You really need to ask yourself this one, as I tell people every time this is asked. Some people are naturals, while others couldn't ride even a bicycle if their life depended on it. Myself, I was into off-roading a bit before I got into street bikes. My first on-road bike? Well, the very Hayabusa I ride now, 17K miles later. I knew what I was getting into, though, and I didn't have to ask a soul if it was too much to start.
As far as 600s and 750s go, every time I see this topic, several people suggest this as a starter bike. Well, they can easily hit 165+ mph and their fairings cost several hundred more than the busa (compare new lowers at $380 for the busa to $475 for the GSXR each side... that's the cheapest you can get them, too ... plus shipping). Their frames are also more delicate than other bikes due to the thinner metal for weight savings. I definitely wouldn't call them starter bikes. A used 250 is more like it.
My advice, then, research your options. Take the MSF course for starters and get your endorsement. You should be able to gauge where you stand after taking this course, too. If you have that natural instinct for bikes and feel comfortable, get a busa, take it to a private road or lot and practice as much as you can. Ease into traffic and keep going back to practice. It took me a couple months to start feeling completely natural with the control (note: not over-confidence, you need to control that and keep yourself in check, don't twist it wide open only to find yourself hot in a corner not knowing what to do!).
Otherwise, if you find yourself not able to quickly react to situations, for instance, you couldn't safely swerve to miss a car that just pulled out in front of you or you wouldn't eventually be able to drag your knees into a sharp corner that just popped out over a hill, then get something cheap and small. If you still find your reactions can't cope, question your place on the road with only two wheels. Getting on any bike, big or small, slow or fast, is a big risk you take. I live in a big city, and I ride almost every single day. I know what it is like to have my life threatened from people pulling out of left turns and right turns, people with road rage and slippery road conditions to top that. I have come very close to losing my life once, too. Remember, being able to control a bike is only a small part of the game as you aren't the only one on the road, and the road itself isn't perfect.