My 2c is this:
I try most racing sims that I can play on my crappy PC, but I always come back to LFS. The main thing I think for me, is the multiplayer experience. It is just awesome. When I first downloaded the demo, way back in 2003 or even earlier, I couldn't believe the immersive feel of the driving model compared to anything else available at the time.
Even now, I compare other models to LFS. Graphics and sound are all very well for some, but for me, I've played plenty of pretty games with awesome sound, but if the car doesn't feel like a car, it doesn't last long on my play list.