Mine "micro-hunts" alot in low light/low contrast situations (concerts) with for instance the 50/1.2. It's like a hyper-active squirrel high on caffeine. This is before the fix (haven't sent it for the fix yet). With the 2.8 zooms, it has never felt especially solid, either.
Get yours sent in. Mine did this before ANY of the mirror fixes. Even if you load up new firmware (back then it was 1.2.3), it would microhunt. After the mirror fix it was very stable.
Best lens to test with the AF would be the 200 /2. Probably THE most sensitive of the entire Canon lineup.
Just thought I'd share - and sorry if this was addressed already somewhere in the thread.

