I'd echo all the safety comments made above. I went right from offroad dirtbikes (CanAm Bombardier 250cc 2-cycle) to a Honda CBR 600 that my bro and I bought from a racer. Not a good move. It was bored out, had steel lines, custom farings, etc. and it was INSANE.
I'm really, really lucky I didn't kill myself on that thing. It belonged on a track, not the streets. I fell a couple times (scrapes only) and my bro rode it much harder than I did. I got some riding tips from friends and I survived the transition, but I'd really recommend proper training and a smaller starter bike. After owning it for about a year and wearing down all the footrests on hard corners, my bro sold it and we both had a strange sigh of relief...
I'd like to pick up a really nice Ducati at some point in my future, but I don't miss bikes right now. I think I'll wait until I'm older and not as prone to late-night figure-8 sessions on off-ramps...
P.S. I also knew a guy who bought a pretty big bike right after graduating high school. He was a quiet, *library* kind of guy. I saw him on the bike a few times that summer. I later met him a few years on in university - he was in a wheelchair. Seems the bike got away from him on the highway and he won't ever walk again. Scary stuff. How come everyone who rides knows someone who can't anymore??