First, I have three disclaimers:

I have not been paid or otherwise compensated for this review.

I have not played the previous Test Drive Unlimited titles, but I did watch videos and playthroughs.

This will be a long read.

I will try to make this post somewhat readable, but I can't guarantee it won't be a mess of things that I wanted to say about this demo.

Overall, I enjoyed my time with the demo. However, I did notice several issues.

Getting the largest issues out of the way first:

The always online requirement and netcode
The demo seems to be somewhat unstable when it comes to online functionality, however, it seems like this is a work-in-progress.
A much greater issue is the always online requirement.
If it persists, purchasing this game will be a no-go for me.
Ultimately, I just don't want this to be another The Crew.

The Graphics
I play on an RX 7800 XT and a Ryzen 5 7500F. On maxxed out settings at 2K resolution with FSR (this game only has FSR 2.0, why not 3.0?) on TAA I achieved a fairly stable 60 FPS. Some areas dropped that down to a choppy 40-45 FPS, but it was never unplayable.

Looking the way it does, it should not perform this badly, but it's a demo.

Frankly, compared to most realistic(!) AA and AAA racing games of the past, say, 10 years(?), the game does not look great. It has its moments. It can look great, especially at night, but generally, it's nothing special.
The character models and animations do not look up to date.
Reading some of the community posts, it seems the way this demo looks is close to the final product.

With that out of the way, let me address everything else this demo had to offer.

Solar Crown's rendition of Hong Kong is genuinely gorgeous. The map is both large and detailed.
When I watched some of the gameplay footage a few months ago, I was worried that the roads would be far too wide, being able to fit 3 cars into one lane. Luckily, this is only partially the case. There are plenty of fun, tight roads and alleys and the game is not afraid of hosting races on them.
I enjoyed exploring, which was primarily driven by the cash, XP and crew rep pickups spread all over the map.
The parking garages are great and I'd like to see the game reference HK's car culture and its frequent use of said garages. Currently they're mostly empty.

However.

For one of the most densely populated territories on earth, it's incredibly empty. You basically don't see people out walking about the city. When you do see people, they're stationary and do nothing.
Traffic is mostly sparse, which helps with navigating the island quickly, but also breaks muh immersion. The traffic you do meet likes to make their tires screech while doing such extraordinary driving feats such as accelerating at a normal rate and turning left. Sometimes they just leave their lanes and go off-road. The taxis are particularly suicidal.
I've also noticed that, when you enter a highway, it's almost empty for the first few moments. Then you hit a lot of traffic, which then slowly decreases in volume.
Traffic is also not particularly diverse and, after talking to some friends who actually live in HK, has cars that would not make sense on HK roads. A Ford F-150, whether it has broken suspension/cut springs as the ones in the game do or not, is not something I'd expect to see in a territory as compact as Hong Kong. The Wuling vans apparently also aren't at all common in the urban areas, if they exist at all.
Speaking of things that you wouldn't expect to find in Hong Kong, according to my friends from Hong Kong: Most things are written in simplified Chinese when they should be written in traditional Chinese.
During my playtime, I saw nothing that points to you being able to buy a house - an important part of the TDU experience.
The Streets and Sharps, or, as I've come to immaturely call them, ♥♥♥♥♥ and Sharts, seem to matter very little when compared to their frequent appearance in marketing.
When it came to picking one, my decision was made easy. The first Shart you meet when you go Sharts HQ is the most stereotypical, obnoxious rich ♥♥♥♥. Why would I join them?

From the few cars I got to test drive, I got the impression that sounds are really quite good. The models seem to be good, with the exception of the 1965 VW Beetle, which just looks weird. The handling on a controller is decent, but still needs tuning. You can't catch a slide beyond a certain angle, as it just won't give you enough steering angle. Drifting is therefore quite challenging. This issue doesn't exist on a wheel.
The cockpit view is inconsistent between cars. The FOV seems to vary. In one car you might only see the steering wheel and parts of the rear-view mirror, in another you might see everything from the left side-view mirror to the shift knob.
Speaking of which, the framerate and detail of the mirrors is abysmal even on max settings. The shifting animation is Forza Horizon 5 levels of lazy. Your character grabs the lever for a second, doesn't move it anywhere, then puts the hand back on the steering wheel. At least go for the Assetto Corsa approach.
You can put your windows down, turn your lights and wipers on/off, put the E-brake on, use the indicators, put the roof down where applicable and change between radio stations, but, unfortunately, none of these actions have animations.
Modifications are Test Drive Limited™ and have a seemingly completely pointless rarity system. When you buy a car, you can configure from a selection of factory paint, interior colours/materials and rims. These options will have a slight and insignificant difference in pricing and, weirdly, rarities. The stuff you see in looter shooters or gacha games. Common, rare, epic, whatever. Why? It serves no purpose here. It's not somehow harder to get or more special. Did an executive see the system when their kid played Fortnite and thought "Fortnite has it and is popular, we need that as well"?
If you then want to modify your car, you go to a workshop. There, you can modify cosmetics and performance.
Cosmetics consists of a livery editor, a total of like 5 aftermarket wheels, each in 3 different colours, window tint and being able to change the interior to another factory option. The livery editor is the only way for you to change the paint of your vehicle, even between factory colours, and it costs you 30,000 currency each time. A 595 Abarth is 30k. You cannot paint your rims, which is bad.
Performance consists of generic power, torque and weight upgrades that do nothing to sound and tires, suspension and brakes. These upgrades have the same pointless rarity system. Better/different upgrades are unlocked at certain levels.
There is no in-depth tuning, you cannot change the stance of your car beyond some arbitrary suspension heights with the suspension upgrades.
These upgrades affect your Performance Rating or PR for short. You'll be familiar with the system if you've played a recent Gran Turismo or Forza title.
It seems to have replicated Forza Horizon 4's charming issue of making AWD superior to everything else.
It might also be a skill issue on my side, but the moment an AWD car, be it a TT RS or Delta Integrale, got past me in my Mustang of equal PR, it was nigh impossible to catch up unless one of the suicidal taxis decided to do its thing.
Either way, the races are actually decently challenging without me ever seeing a difficulty setting. They are also a pain in the ass to start, as all of them are always online and CANNOT BE DONE AGAINST AI BY CHOICE. Each time you start a race you are forced to sit through a 30 second waiting screen while the game looks for other players to join your race.
If you make a critical mistake, you can choose to abandon the race, but you cannot restart it immediately. So you abandon it and start the race again. Sit through that waiting screen again, for nobody to come.

• The economy feels well judged. Not too grindy and things aren't thrown your way for free at an exorbitant rate. Except for the price of the 2009 R35 GT-R. Does the person responsible for the pricing own one?
  
• The photo mode is ass.
  
• As mentioned above, the F-150s patrolling the streets all seem to have collapsed suspension.
  
• When you turn off your engine, the gauge cluster and other screens stay illuminated, implying that the ignition is on. But the radio no longer works. Every car I've been in didn't need ignition or a running engine to have a working radio.
  
• During heavy rain you literally CANNOT see out the windscreen while driving in one of the cockpit views.
  
• The exhaust gas visual looks terrible.
  
• There are audio pops or glitches occasionally. One I could reliably replicate happens when switching between certain views.
  
• I somewhat doubt that a stock Alfa Romeo MiTo Quadrifoglio is capable of spitting flames. Also, the interior seems to be offset to the right?
  
• The speed bumps make your car jump even at low speeds. It's like they're boxes, not semi-circles.
  
• There is no wind noise when driving with open windows, regardless of speed. Might be intentional.
  
• The game does not remember which controls preset I was on upon restart. Luckily it does remember the preset itself.
  
• Please let us skip the cutscenes that you get when leaving your apartment or really anything involving your character with a toggle in the settings.
  
• The road discovery system is inconsistent. Sometimes a 300 meter stretch of road gets marked as discovered by driving 3 meters down it, sometimes only a tiny segment gets discovered. The greatest crime? Roads aren't counted as discovered if you drove down them while test driving a car or while racing. Why.
  
• A teensy tine itty bitty issue I forgot to mention earlier. Each and every car I could get into at a dealership is Left Hand Drive. In a place that famously drives on the left side of the road. I get it for some cars, where a RHD version wasn't available, but a Nissan 370Z? A Jaguar E-Type?
  
• And seriously, what is wrong with the Beetle?

I hope this review is somewhat helpful. It ended up a tiny bit longer than I had originally planned and I probably forgot some things.