Fun, but can get old after a while.
Okay, now to the serious gameplay. When you start a new game, you'll pick a character, and an opening scene will show you arriving in Honolulu. Beyond that, there is no story whatsoever, except for one you make up in your head. (See parents? Video games don't destroy creativity!) After you land in Oahu, you'll choose a car and house to put it in. You can own as many cars and houses as you like, given that you have the cash. Obviously, you get cash by doing events and missions. Both the events and missions are fairly straightforward, races having a goal of crossing the finish line first, time trials having you get the best possible time, and speed challenges have go as fast as possible, and get scored for it. Mission are simple. Drive to a destination within a time limit. However, the game can throw a bit of monkey wrench into a seemly easy task. In most missions, there is a "Driving" meter that penalizes you for bad driving. This was clearly to add some challenge into the missions, but it usually turns into frustration, as you have no choice but to drive dangerously to beat such tight time limits (For example, you have to drive a model two miles in one and half minutes in heavy traffic). The meter also unfairly penalizes you for lightly tapping another car, or having one wheel touch dirt instead of road for less than a second, but doesn't penalize you for driving 200 mph, or slamming into a wall. Missions also can provide good bonuses for "perfect" driving, so it's frustrating to have the meter drop one point, therefore costing you the bonus. Having the police get in your way is also a pain the ass. Still, the missions are fun for while, and can get you some good money and cars, but they start to get tedious after a while. In fact, if you choose not to play online, playing turns into grinding out missions for cash and cars, and it gets plain boring. It's still fun to amass a huge collection of cars, but once you get the best house and best car, there really isn't a reason to buy more stuff.
If you do play online, you'll find some decent user generated events and new cars to buy. Plus, it's fun to race other through the city and freeways, but even that can get old after a while.
To wrap things up, Test Drive Unlimited is a decent and ambitious game, though some of the ambition falls flat. Despite a few problems, I'd recommend this if you like open world games and/or racing. (Another option is to wait for Test Drive Unlimited 2; it's been confirmed to be in development.
The Good:
Open game world that fun to drive around.
The recreation of Oahu gets the job done.
Huge variety of well rendered cars.
Can own houses and collect cars.
The Bad:
Missions can be frustrating.
Repetitive buildings and cars.
Lame "towns" outside of Honolulu.
Not as immersive as one would like.