Being a Twisted Metal fan, I was somewhat disappointed with it. The biggest problem with the game is that the whole thing seems to have been undertested.
I can't comment much on the multiplayer because I rented it in the first week. This was when the multiplayer was broken and, by broken, I mean "Duke Nukem Forever broken". Since the game needed an online pass and I was just renting, that didn't bug me. It's been patched since then so it is actually possible to get an MP match. However, it is a symptom of the undertesting. (And I find it difficult to believe David "I don't believe in large betas" Jaffe when he claims innocence on this.)
This affects the single-player as well.
On the Mr. Grimm race, if you don't hit the hill when leaving town correctly when you're driving at it, you won't make it up the hill.
On the Dollface boss fight, when you start the round, there is a good chance that one of the escort limos will ram you before you can even move. If you're in a light-medium weight vehicle, this will send you sailing off the cliff with a 50% health penalty. (The Dollface fight is difficult and in a largely tedious way.)
As has been said, the cars will all come after you which (to me) isn't a problem in the Juggernaut/arena fights. However, when you're in a race event and half of the cars + Road Boat (He has a magnet on his car that, if he hits you, he'll turn around and fire you towards the back of the race which, on average, drops you from 3rd to 11th place) are attacking you while the front 5 stroll their way towards victory, that becomes less a question of skill and more a question of luck and maining the fast cars. In fact, the races are pretty much where the difficulty spikes occur as the game simply isn't made for racing.
The list goes on. But is it bad? Well, not really. There is actually a lot of good here and there is a solid foundation for a sequel if they removed the races (does anyone actually like those?), and did a lot more testing before release. In many ways, calling it Twisted Metal is appropriate as the first game was ok but the series really took off with the second game. Obviously, given that it didn't really do that well in sales, whether this iteration of Twisted Metal gets a sequel is probably up in the air.
As for whether you should buy it, I would say the question hinges on whether you were a fan of the series. If you are, it's probably worth $30. $40 if you're really into multiplayer. If not, then rent it and see if the multi does it for you and/or you can get past Mr. Grimm's events without screaming.