This may be a late review, but it's worth reading.
Overall the game is highly addictive and worth a quick rent because if you are not having fun in the beginning of the game then you will not like the entire thing because it is very repetitive. You win races and break milestones while driving up your bounty in order to race the current challenger. You do that basically 15 times and then the game is over. There is replay value in the challenge series but it is more of the same there too.
The most annoying part of the game is the fact that you have to throw yourself into police chases and almost beg for the cops to stay on you long enough to get a high bounty and complete the milestones. If you are a good driver this gets very boring and can make it seem like way too much work just to challenge the next racer on the blacklist.
Rent first, if you like it, then buy because there are hours of the same fun ahead of you.