Damn this place seems pretty inactive... I am new to this forum but not to riding so I'll throw in my two cents.
I, too, started on a 250. The bike I went to after that was an 04 zx6r and it took some adjusting.
When you say you sat on the R6 did you mean the R6-R (supersport) or the R6-S (sport)? There is a HUGE difference between the R6-R (supersport) and the 650 bikes. The 650's are defined as a "sport" model which means they aren't as aggressive as the 600 supersports (R6-R). The R6-S is basically the older model R6 that is updated in some areas but ultimately tuned down so as to keep the power in a favorable range for riders who don't want the insanity of a supersport.
What you should know about the R6 -
1. It's one of the worst bikes to work on. In fact it is THE worst bike that I've ever worked on because of how inaccessible Yamaha makes everything. I had to drop the radiator (not remove it completely though) just to do my spark plugs.
2. The Powerband is a little lax in the low end. By nature. This actually works out well for transitioning riders, but I had to change out my gearing a little bit because it felt like I was riding a 250 right up until I hit 12k RPMs and then the thing takes off like a banshee. This is because the bike is designed to be ridden on race tracks and not in suburban roadways.
If you are more suited for low-mid range power then this might not be the best pic for you (that is, assuming we are talking about the R6-R).
It is very nimble in corners and of the sport bikes I have, I tend to corner better on that bike with the exception of the throttle "dip" between 6-8k rpms. This makes it a little twitchy if you aren't precise and smooth with throttle engagement but it's really only problematic when you are leaned over... then it makes you pucker a little bit.
Longevity: this is my biggest issue with my R6 (07). I am not sure if this will be your problem or not (if you are eyeing a brand new one) but that year of R6 is notorious for having these phantom issues where it dies and doesn't want to start again. I've known of at least 10 other R6 owners who have encountered the same problem and have yet to actually isolate what the problem is.
I love the handling and the design of the R6 but as an owner/mechanic/avid rider I'm not too impressed with some of the other "quirks". I am hoping to resolve these issues so I can go back to riding it though because they really are quite awesome.