I started with a Ninja 500. They're GREAT bikes. Fast, nimble, forgiving, easy to learn, and they look a lot better in real life then they do in pictures.
After about a month of riding I was confident in my ability to ride something bigger, and have ridden something bigger since then... but I'm still a firm advocator of starting out small and working your way up. That and the 500 got me 55 MPG, which was awesome!
You'll get people on here that will tell you starting out on a 600 (Or even bigger) is okay, but IMO, that's just dumb. They say all it takes is responsibility and that you should just never roll back the throttle and you'll be okay... But there's a lot more to learning than throttle control.
The 600s are less forgiving, nimbler, lighter, faster, and just a generally worse bike for beginners. The jump from 500 to 600 cc's may just be 100, but the difference in power and performance is like night and day.
I never got bored riding my 500, it was always a thrill. Eventually even a bike as fast as the 500 (Which is damn fast compared to nearly any car) feels slow, and you want something faster, but I could still see myself having a 500 as a daily rider in the future at some point.
I highly recommend them.
-Gary