I've done about half of the races so far. I enjoy them, it's a nice change of pace sometimes to do a quick race here and there.
My only complaint is the checkpoints. I wish they didn't have to 'explode' every time you go through them. It's just obnoxious, especially when playing late at night, and I really don't want to turn the volume down, but this overly-done BOOM every time I go through a checkpoint just gets annoying.
Edit:
Also, in other news I just got a new PC.
Intel Core i5 quad-core @ 3.2Ghz
8GB of RAM
1.5GB video card
1TB HDD
I'm considering double-dipping just because of all of the crazy mods for this game, just not sure yet. I've put 65 hours into the PS3 version and not sure I want to start over again, or if I would even feel like sitting at the PC to play it when I've got my surround sound and all that in the other room. :/