What you've come to expect from the Wipeout series, only much, much prettier.
At $20, you'll certainly be getting bang for your buck; the game's relatively lengthy campaign has a high learning curve meant to extend the life a bit. Completing the campaign is no easy task; levels were designed very well and so that memorization is required for success. Wipeout HD's longevity is pretty damn high for a downloadable title.
On the flip side, you won't really spend much time online, which is far too simple for its own good. The only game option is normal race mode (tournaments are basically a large number of races), and its near impossible to find a full match. Cars are also pretty unbalanced, and while the car 'Icaras' is often cited as a 'newb car,' this is solely because it is much better than every other car in half the tracks. It would have been nice to have some variety for competitive play, but as it is, online is merely an entertaining distraction for a short while.
Single player is far more entertaining. There are plenty of rewards for extensive playing, including new HUDs, ship skins and trophies; and while some of the trophies are virtually impossible to get for any but the truly dedicated, a good amount are fair and challenging enough to compel the player to succeed. Mission variety for the 'campaign' is nice, and the AI is perfect for each level of difficulty; levels are designed very well, and the more you play the more you succeed. Weapon and boost pad placements are close to perfection.
Weapons are generally well balanced, although they feel a bit weak and uninspired. Still, it's nice to have them there, even if they throw a huge amount of luck into the pot. The skilled player isn't necessarily the victor.
The most distinctive aspect of Wipeout HD is the Pilot Assistance feature. This system makes it so that your ship shies away from walls and generally makes crashing a less frequently visited chore. The good news is that this system is great at aiding newcomers and makes it so that even without skill, any player will be able finish a race (you can hold X down the whole time and that will work; just don't expect to get good scores). Feel free to hand a 5 year old the controller; the game is forgiving if you aren't trying to conquer its trophies or later campaign levels. On the opposite side of the spectrum, the system is slightly flawed; when items or boosting pads are placed close to the edge of the track, Pilot Assistance system often forces your ship away from them. It's not a major flaw, but it's not easy to ignore, either.
Whether you like the series or not, though, there's no denying this -- Wipeout HD is a gorgeous game. Running at a flawless 60 FPS and 1080p, this is as technically proficient a game I have seen on PSN; the cars, tracks and special effects are all simply beautiful to say the least. Screenshots can't replicate what the human eye can; Wipeout's easily one of the best looking games on PS3. The soundtrack doesn't fare nearly as well, but it's competent.
What it really boils down to is whether you want a good, solid traditional racer, which defines Wipeout. As far as futuristic racing games go, Wipeout HD takes you on the beaten path. A beaten path running at 60fps and 1080p, but a generic, beaten path nonetheless.