What kind of documents do you want to scan? book pages? business cards? 8.5" x 11" single sheets? larger single sheets? double-sided single sheets? photographic slides? What?
Personally I have a few Canon Lide u660/u1200 or something scanners that I've scavenged over the years and use with Vuescan scanning software. I haven't used it since 10.7 but I'm positive, given the the developers propensity for new releases, that 10.8 won't an issue.
I think that the Fujitsu ScanSnap S1500M is the gold standard of document scanners for the Mac. Search for the numerous posts in MacRumors to get more information. A+++
Instead of a S1500M, I went with the S1300M. I'm glad I did! It's portable enough to put in my book bag and take with me to school if I need to (done this more than a few times), can be run and powered using only USB (this is the very method I use), it's fast, light, folds up small... it really has no cons except that it could have been cheaper. I still feel that the S1300M has a fair price though.
I have scanned over 45000 pages with my Fujitsu Scanner, both single sided and double sided. I have scanned some documents 500 pages in length. The Fujitsu just runs right through them. If you had pages stapled together you need to take some extra time to make sure the pages aren't stuck together, but other than that it just works.
I bought a Scansnap 1500m based on comments in these forums and couldn't be happier. Scanner is great and included software works well. In the month that I've had it I've scanned over 10k pages with only a few jams and errors.