I ride both, and I hate to say it, but am slowly coming to the conclusion that my 30 pound $600 bent is a better bet than my 18 pound, $1800 Cannondale (which I love).

Why?
1. Aerodynamics. Under equal conditions -flat or downhill, the bent is faster. I've hit 43 mph downhill on the bent in places where I could barely break 40 on the C'dale.
2. Comfort - as noted in another answer, it's easier on the neck, wrists, and yes, butt.
3. Fun - people look at bents and smile/wave.

Advantages of an upright?
-More nimble, manuverable- important if you have to deal with traffic and unknown roads. I'll ride my C'dale anywhere, anytime, with the knowledge that I can ride out of any situation. I only ride the bent on roads I know.
-Easier to transport, especially by plane.
- You don't get the smug looks from other cyclists that you get when riding a bent. (As if to say, you aren't a real cyclist on a bent).
- Uprights are better than bents on hills. This can be important.