Auto-focus in the dark on the 5Dmk2 sucks. The only reasonable way to fix it is with a device that does AF-assist linked to the camera's focus points (meaning, "blinks a red grid when focusing").
Flashes like the 580EX do this, but they're huge and since I don't actually use the flash, I don't like lugging around something that doubles the size of my camera.
I have a Speedlite Transmitter ST-E2, which would be perfect if not for it's completely retarded choice of battery technology. It uses 2CR5 batteries, which have horrible battery life, and for which no functional rechargeable version exists (I've tried several, they just don't work). So in reality, I can't use this thing because the fucking battery is always dead whenever I need it.
I bought a Speedlite 270EX II because it runs on AA batteries, only to discover that it's useless because it doesn't understand focus points: it does focus-assist by firing a blinding white light that lights up the whole room. If you used this device at a show, you might as well say "please stab me in the face".
Is there really nothing small that does focus-point-aware AF-assist that doesn't use stupid batteries?
I've asked this before, but time has passed and maybe there's a better answer.