A friend of mine just picked up a 600 RR and I posed him the same question (he used to have an F4i). Just outright, it's a different monster. Rough overview...
inverted front forks
about 10lbs heavier
close ratio transmission (he says shifting gears is awesome!)
very aggressive riding position (clip ons and rearsets) - he's about 6'3" and feels like he's sitting on the gas tank. His arms rest on his thighs when he's riding, but he's actually very comfortable on it. No aches or pains... numbness in his legs, but he hasn't ridden it a long distance yet.
He's still breaking it in (only 100 mi. to go!)
throttle response is different than the F4i
overall he says it just feels different than the f4i (his gf has an '01 f4i) but he thinks it's great.
Hope this helps