Yeah...the bigger, full-frame sized, all-glass pentaprism viewfinder image of the 5D series cameras is a step up from the little,small, pentamirror Rebel bodies and their squinty viewfinders. For some people, just having the larger, clearer, more-contrasty viewfinder image improves their picture taking. I think this is especially true for people like me, who wear eyeglasses.
I dunno...the 5D-II is fairly easy on lenses...it's not "stressing" lens performance the way the smaller 1.6x Canon high-MP sensors do...every single Canon lens I have seen tested for resolution has delivered it highest resolution figures on the FULL-frame Canon's--and NOT on the crop-body cameras....so...don't worry all that much about lens quality with Canon EF lenses on the 5D-II...even with crap glass, it'll perform better on the FF body than it will on the 1.6x Rebel you're used to.
Buy the Mark II. Enjoy it! Revel in it! Sleep with it! Go out and get drunk with it. Introduce it to your friends! Write it love poems....no, wait,wait, maybe I need to back up a bit...