Also, it seems that a lot of green drives over 1TB seem to be failing prematurely, at least in the WD line. I haven't had any problems myself, but have seen quite a few people writing in these forums that they have had issues.
My 1.5TB and 2TB WD Green drives are at 12500 hours and 9000 hours (approx) now and have had no problems at all. Based on anecdotal evidence around the web they don't seem any worse than any other hard drive for reliability.
Performance-wise they're not an ideal OS drive but no mechanical disk is an ideal OS drive