I heard about LFS a long time ago, but did start playing it until not long ago. Here's how I started:
I used to play Racer since it's free and I wanted to try something besides Gran Turismo. I stopped playing after 0.5.0 because my computer couldn't play it back then. Even I got another computer that can run it some time later, I eventually quit because the downloadable content aren't so good (missing textures in a track even it's completed, for example) and its "collision detection" is very bad (drive into a wrong place will send the car up in the air in unrealistic means). Around that time, I heard of LFS. It was still S1 back then, but my computer isn't good enough to play once again. I was really amused by the video "South City Robbery" (you guys may remember that video). South city Cab Co., which is a team that still exist, was part of the cast.
I get bored with GT4 pretty quickly as there are too many events to complete and the "driving mission" is driving me nuts (can do any of them except the last one). I soon rediscovered LFS. Spent some time on the demo and really liked it. Then I decided to get S2 and never regret it. I started at about Patch X when you can get fault start.