I've toured on drops, and on flats with bar ends. I currently have butterflies on a bike as well.
I personally prefer the drops, as I find them most comfortable. I do ride this bike more than the others though so it may just be what I am used to. Shifters can be more expensive as well. The big advantage that drops seem to give is the chance to get down out of the wind when it really picks up, and I prefer being down on the drops for decending to other positions on other bars.
Early reports on the butterflies is that, although they don't offer as many useful bar positions as you would think they are pretty comfortable. They do seem to make the bike feel more sit up and beg. I'm finding it difficult to get the bars set up in a way where the both sets of flats are useful at the moment. Plus side you can fit much cheaper flat bar brakes/shifters, and it's easier to set up with v-brakes.
Flats were ok, but after a while I found my hands started to feel less comfortable. I spent almost all my time on the bar ends, which is why I decided I might be better with drops.
I haven't tried the Midge bars.
