I can't imagine riding with anything but a drop bar for more than half an hour or so.
Flat bars, or nearly flat bars, are acceptable as well, but not for much longer.

When I'm in the drops, my hands and forearms are at their most comfortable and as relaxed as possible while still holding on. "Regular" bars, like the ones posted earlier in the thread, make my wrist, forearms, elbows and shoulders operate at very uncomfortable angles. They also provide poor and unstable manoeuvring.