Mediocre single player is a means to the amazing multiplayer end.
But then I was hit by a mortar round, which lead to me seeking revenge, which led to further retaliations, which led to me being taken out by an air-to-ground missile. Basically, this game is transformed by multiplayer, the agony of inching slowly off the bottom of the screen is as indescribable as the glee you get 5 seconds later when the leader is ducking and diving as your targeting reticule tracks him relentlessly. The action (and it really is worthy of that term now) is fast paced, broken only by the brief pauses to load a new track (which more often than not are used to tally up how many 'favours' you owe your opponents). There are more than enough tracks to keep you happy and your sure to find one or two that you'll play endlessly. In multiplayer the simplicity of the handling and weapons actually enhance the game because you can concentrate on revenge. Sound is not much to shout about, but again the best is in multiplayer; namely the 'hissss' of an incoming (but off-screen) missile, which is one of the best sounds I've heard in a game for provoking panic.
Put simply, had this game been shipped without it's single player element and just had all the tracks and cars available from the outset and it would have lost nothing. You will spend a few hours on single player unlocking everything, then plug in the multitap and never look back, I know I haven't and I've had the game for a year.