With a terrible career mode and an online experience that isn't much better, you'll want to steer clear of Blur.
Its "Career" (single-player) mode is trash as there's no real balance in the difficulty at all: Easy is so easy it'll make you want to cry, and medium is so frustrating you'll want to put your head through a wall. Double this if you're trying to go for 100% completion in the game, which requires you to complete certain specified tasks in each race. Sure, slaloming through a series a gates is fine, but it becomes near impossible when the A.I. is hitting you with everything it's got and constantly knocking you off course. Even just finishing the race in the top three so you can progress can be more work than it should be. Having to repeat a track a couple of times to beat it is no big deal, but having to repeat a track 15 times just to get past it is less than a good time.
Multiplayer comes with its own set of issues. While playing against other live competitors should be outrageous, it instead dissolves into total chaos as soon as a race starts. You end up with 10 - 20 racers close together, hitting each other with every power-up they can grab, and your car ends up wrecked before even the first lap is over. What's worse is that Blur's online system doesn't match players of similar ranks together; level 12 racers end up racing level 50 racers. So even though leveling-up should be addictive, it ends in just as must frustration as single-player because the higher level players have much better vehicles and access to mods that make their cars even more superior. You spend hour after hour of having your butt handed to you before you actually obtain enough equipment to fully compete, and it's not enjoyable enough to keep at it until you reach that point. Even if you do, you'll have to deal with other glitches such as power-ups not appearing until after you're already past them, lack of track variety, unresponsive controls due to lag, and disconnecting from the game servers entirely.
The underlying problem with Blur is that they didn't take enough time to balance everything out, and it's hard to recommend this game to anyone except for the truly hardcore racing fan who needs something to pass the time. Many gamers will find the obvious lack of polish ruins the experience for them, and taking into account the decreasing number of gamers playing this online (its online community has already diminished to about 1100 users playing at any given time as of the date of this review), most will be better off waiting for Need for Speed: Hot Pursuit, or maybe trying another recent racer like Split Second.