I use drop bars now, because I like to vary hand placement a little on longer rides, but I find that having the bar about the same height as the saddle is ideal. If the bar is very much lower, it's not as comfortable or easy to look around, and, at commuting speeds, air resistance isn't as important as these other two factors. If the bar is a lot higher than the seat, it's pretty darn comfortable, but, for me at least, your center of mass is too far back from the front wheel, which makes it slightly harder to control steering in rough patches, which makes me a little nervous, to be honest.