I also agree that I wish I had started on a 500 rather than a 250. I bought a Ninja 250 back in October and just upgraded to a zx6r. My friend that I sold the 250 to also says he can't wait to upgrade. Had I bought a 500, I still would have it, and wouldn't think of upgrading til the end of the season.
However, I do say that owning the 250 was crucial for me. I think I wouldn't have been able to handle the 600 without something smaller first, or at least would have scared myself into riding like Grandma very quickly, especially since I hadn't ever ridden before. I had ridden a dirtbike once and a quad once before buying my 250, and the first week I rode the 250, I was hitting full throttle all the time and begging for more.
I'm also a big guy, weighing in at 240lb, so the 250 was a bit small for me anyway. Great bike to learn on, just too small for my size if I want to do any highway or distance riding, which is why I upgraded. And a 250 will lift the front wheel if you shift hard or dump the clutch, so don't think its a completely boring ride.
And if you're into cars, my 250 ran dead even with a 2000 Volkswagen Golf GTI 1.8 turbo that had minor mods, and my 250 was stock except for cut down intake snorkels