There's been quite a debate in other threads about this. Actually I couldn't properly say that I tried drops out, because I've been using them for years since my first bike, a roadie, back in times when the only decent bikes available were roadies. Five years ago I did give flats a try because a friend of mine advised me to do so, and found them tiring for my back in long trips and provided very poor slipstream against the wind (it gets quite windy at times over here). With the drops, my back is relaxed with my arms and hands supporting the upper trunk weight, and the wind just breezes around me. My whole commute is 6.5 miles one way (4 open road, 2.5 town), taking 19-22 minutes depending on wind and temperature and I ride all the time in the drops. On tour trips I go 75% drops and 25% hoods I'd say. Many people would tell you different experiences, but for the time being, drops are the only choice for me.

PS: That is not an MTB, but a hybrid, although also in the shed, an MTB-tourer, with drops as well. I'll post pictures of that one soon.