No problems at all, I do it all the time. That 80/1.9 is gorgeous, and all my Zeiss Pentacon6 and Takumar P67 lenses fit via adapters too.
a) you only get to use the centre-spot metering, not full-area.
b) there's obviously no AF, but the front/back focus arrows still work if there's enough light and you're wide-open
c) you have to do stop-down metering. If my M645AF has taught me anything, it's don't trust the TTL meter at all, unless your eye can form a complete seal around the eyecup stray light gets in and blows it out by a few stops (even more if you're using stop-down metering with NDs or CPLs).
d) get a handheld meter anyway