I'm building a single speed 29er, and I need to choose 175 or 180mm cranks. I've read some of the discussion and understand that the longer crank may help with leverage. I have 175s on all my bikes but one, and I am leaning towards 180 for the halibut. If it matters, I'm 6'.
I compared the pedal to ground clearance with my 26" 175mm crank K2 and this 29er, and I'll gain ~0.7" of clearance even with the longer cranks.
Because ther aren't that many choices for 180mm length cranks, I run standard 175mm cranks on all of my bikes (including my single speed 29er) except my fixed gear road bike, which runs 170mm cranks for a little extra ground clearance in corners. Running longer cranks might work better, but I doubt there's that much of a difference. But anyway, I'd suggest asking this question in the MTBR 29er forum - there are likely lots of people there who have used longer cranks and have something to say about it.
Everyone's experience can differ, but I found 180 was noticeable. I ride a Haro Mary SS. I really don't notice the difference spinning on the flats, but climbing is where I found it to be a plus. But then that also could be attributed to the 29" tire, as I have not ridden a 29er with a 175 crank.