Personally, I started riding on the track on a Gixxer 600. I had an R1 street bike but wanted to start on something smaller. So i found that Gixxer and bought it from a racer. It was already track prepped and ready to go.
Saturday was fine. In the last session on Sunday i was taken out while passing somebody on the outside in a pretty fast turn (he lowsided and his bike slid into mine). To this day that is the worst crash of my career. It resulted in me being unconscious for 48 minutes, plus a broken collarbone, 2 broken ribs, torn ligaments, brain swelling, etc. That is why i hate it when people skimp on safety gear because they will just be "taking it easy" or because they are "just riding in Novice". The worst crash of my career happened while riding 10 seconds off of a race pace, in the Novice group.
Anyway, the bike was totalled. Instead of buying another one, i just started riding my R1. I found myself thinking "no way should that guy be passing me on that bike". It didnt take me long to realize skills are much more important than the bike.
So i parked the R1, got another 600 and spent some time doing lots of track days, several track schools, etc. Fast forward some years and now when i get on a 1000, it is a different story. There is something to be said for "riding a slow bike fast" and it is great being able to lap most of the Advanced group on a 600. There is nothing worse that a Liter bike "straightaway hero" who makes all of his passes on the gas on straights. But with that being said, there is nothing more rewarding than riding a "fast bike fast".
Few things are as difficult and as rewarding as riding a Liter bike at race pace. Once you get to racing a Liter bike, everybody is fast. There are no "straightaway heros" because everyone has a lot of power. Contrary to popular belief, when actually riding a Liter bike fast, you have to carry good corner speed, get on the gas early and hard, push your brake markers and do everything else you do on a 600. The difference is that you have to do so while the bike is trying to rip your arms off and you have to not only manage wheel spin, but wheelies also.