I know the few things that will not work:
1. the battery;
Leping,
The 5D batteries (BP-511A) will not work in the 5D2 at all? Or will they work, only without the "power-remaining" and battery I.D. functions?
Thanks,
daws
EDIT:
@ dwdallam -- francois' comment above is correct, with your eye well down into the cup, the Canon Angle Finder C gives a full image of the viewfinder, including the shooting info readout, at 1.25x; a circular crop of the scene at 2.5x. (My angle finder on/angle finder off routine is purely personal idiosyncrasy, from long habit of looking straight through the viewfinder to compose. I've tried leaving the angle finder attached, but composing through it just never "stuck." Not a problem with the angle finder itself, however.) At 1.25x you'll notice a slightly darker image, a bit of light dropoff on the periphery, and a bit of barrel distortion, but otherwise a good, strong image. At 2.5x there's no doubt about focus.