I used to run an '07 CBR1000rr then switched to an '07 GSXR 750 followed by an '09 GSXR 750 mainly to have something more nimble than the old model CBR.
As you probably know, the 750 is a 600 frame and parts with a bigger capacity, so to me it was like having a worked 600 race bike without all the cost involved of building a works 600.
They where both fantastic bikes BUT...
My gen 4 feels as nimble as my GSXR 750's were, and that really suprised me, so the big benefit of the 750 handling is slowly slipping away and these days weight wise there's nothing in it between the 750's and 1000's.
Don't get me wrong, the 750's are plenty fast but as you do them up the good power seems to creep higher up the rev range and the window of needed grunt (against thousands) ends up in a small aggresive little window which makes it harder to ride fast.
If you can keep it singing and right in the top of rev range you'll love it, but the 1000's have a wider range of power. You'll be working harder than a 1000cc rider on a 750 to, as you'll be working the gearbox to stay in the good part of the rev range, which for an up and coming racer can be a good learning tool, so that's not all bad.
I loved reving the snot out of my 750, trying to stay infront of mates on thou's was a big challenge. I'd say it made me a better rider than I otherwise would have been as you have to make up the difference in braking and corner speed, they require a little more aggresion to get the most out of them.
Because the 750's made less torque than a 1000 they also used to give a bit more warning before the rear would let go, which was a welcome change to just being flicked off the 1000's, but now with traction control on the thou's and tyre tech catching back up to the power these 1000's are trying to put down it doesn't seem to matter so much these days.
Basically I'm glad I owned them, they did my riding craft some good, but really I'd buy a modern thou over a 750 if you are already experienced, I'd say the 750's are perfect for someone looking at a 600 that wants more useable power than the fluffy 600's and wants something they can grow into a little instead of scaring themselves with a thou.
If you can find a cheap good value 06 or up model, which where the first with the slipper clutch, I'd still buy it just new for new I'd pick a thou.
Hope that helps.