Rfactor is fun and has some cool mods, but it also has some major problems IMO
The physics just don't have the "feel" that LFS does, I can feel the car starting to lose grip in LFS, but not really in Rfactor. I'm not saying LFS has more realistic FFB, but when there is no "seat of your pants" feeling the accurate FFB of LFS really helps IMO
It's nearly impossible to find a server with people playing the particular mod you want to play. You can just play a wide variety of mods of course, but then you'll end up downloading 20GB of mods. This is the biggest problem with the game IMO. Maybe it's just because of the time of day I play though.
It has some major stability problems too, especially with certain mods. Some of them flat out crash my computer (and it's not my PC, i can play new games like Crysis, COD4, etc just fine).
I'm not slagging Rfactor or anything, I just prefer LFS. While it doesn't have as much content (in terms of tracks/cars), the content it does have is of great quality, and most importantly I can easily find and hop on to a server for some races.
That being said, some of the mods in Rfactor are very cool, being a single-seater fan I find the 1979 F1 mod to be really nice (all those old tracks and awesome sounding cars). Nordschleife is always nice too of course