"It depends" is probably the right answer. The only certainty is mileage in the low 20s until the engine warms up, so if your commute is only a couple of miles and that's all the driving you do, you are better off with a conventional car. You'll get the most improvement with long trips, or at least short stops with lots of mileage (delivery trucks would be a great application for hybrids). It can perform much better than a conventional car at low speed -- I've found that when traffic is bad and I'm crawling down the highway that I get better mileage than during non-rush-hour, low traffic times.

Other than that, conventional wisdom applies -- you will get lower mileage in cold weather, and in hot weather with the AC on, and at high speeds, and with a lead foot.