I flipped the stem on my 2007 Specialized Stumpjumper HT, with a flat bar. The change is fairly significant - more so than when I flipped the stem on my road bike.
When I flipped the stem on my road bike, I knew why - I wanted a lower position. I've never thought about doing so with my mountain bikes - until yesterday, I'd always left the stem alone.
While the bike looks a bit strange with the sharply down-angled stem, and while I'd have to say that I felt a bit more stretched out, I also liked the feeling on the well-graded, short, flat dirt road I took yesterday.
So what advantages or disadvantages might come with flipping my stem down?