Only 2 problems I have come across with the 64bit version,
Gb LAN appears to be running half-duplex (apparently this is something which can't be fixed with the config settings, yet!) but jumbo/extended frames are working OK.
Under compiz when playing a podcast from disk and browsing, Firefox goes into grey mode and hangs after some time, looks like a memory problem but the system monitor doesn't show anything abnormal.
Turning compix off fixes the Firefox problem and puts the system into turbo drive, I think I prefer the speed boost to the special effects.