oh yeah. last summer bought a $400 325e off a gf's friend. POS. put $100 in it to get a working radio, wipers, lights, etc. This was when I was really excited about tracking to the extreme and having a soft "daily". Yeah, well, turns out after the new car fun wore off, I missed the fun one, and I didn't see the point in stripping the car and would rather just make my red one get better gas mileage and be more comfortable... but i miss having a 4 door. Not worth having two cars in the driveway or the insurance, and yes, my mom was right and I was silly.
Also, bought some Hankook R-comps used for cheap off of a friend. Then decided it was not worth the hassle after attending an Auto-X with my friend Will and running them. half hour before and after, lugging around and then later storing tires? No thanks. Sure they were sticky, but not that serious about it. Azenis are still a lot of fun and grip great. Sold them to my friend Dave after hassling him to make sure he was ready for R comps.
So yeah. I learned the hard way that what you do to your car doesn't matter as what you gain in experience with it. It wasn't that expensive of a lesson, although it wasn't cheap. And personally, it never clicked until the classroom session after a few times on the track.
Not claiming to be an expert, hell... I'm still learning definitely. But there is things I can say to try to help and inform those not yet aware of it themselves.
http://www.youtube.com/watch?v=5E80OeyKoRo E30 325is with 300/475 GC, no rear bar and 205 Azenis sport tires. Stock power (~140rwhp) and 3.73 diff.