The most fascinating part about SLR:R is that in spite of all the bugs, glitches and lack of balancing, it's still fun.
In some ways, SLR:R is the perfect street level racing game: You really get to build up your car. It's not about unlocking features and levelling up as in NFS:Underground: You make money to buy spare parts to boost performance and (usually) replace the parts you've blown or destroyed. The garage-part is much more nerdy than the arcade-feel of most other games in the genre.
There's more fun inside the garage than outside, however. The racing never gets as fun as you could hope, so you're likely to focus more on building the perfect racer than actually racing it. The graphics are pretty bad, the steering could be better, and the game is badly balanced. Getting far in the game is pretty easy: Buy a small car, strip is down to the bare essentials (including removing all panels, windows, seats, etc.), sell the spare parts and use the money on engine parts, wheels and suspension. That should buy you a super fast racer that can outrun almost anything on the drag strip.
In short: If you like fiddling around with engines, transmission settings, etc., there's a lot of fun to be had (Don't expect the game to give you too much help tuning the car, though). But if you're looking for a fun street racing game, get NFS:Underground 2.