If you're heavy on the gas, follow close, pass, stay on the gas until the last minute before stops, etc...you can prob. expect something in the low to mid-40s for mileage.
You may find that your driving style changes a bit with the Prius as you start to play the mileage "game" and realize all that 'hard on the gas' thing does is burn gas and doesn't get you where you're going significantly faster while increasing the risk of an accident. But you also may not change...everyone reacts a bit differently. I definately went from serious Type-A driver personality, always 10mph over the limit, 'get outta my way' kinda driver to a very speed limit following, easy on the brakes, easy on the gas "how high can I get my average MPG this trip" kinda guy.
I'm hoping that by the time I have my next Prius, someone will have figured out how to read the mileage info directly out of the Prius' computer, so I can tie it into a PC with GPS and have ultra-detailed, mile-by-mile information. (Hey, how good did I get going up Mt. St. Helens? How about back down? What variance is there in MPG when I go 65 down a certain stretch of road vs. 55? What difference does temperature make in a certain stretch of road at the same speed?) Heck, if you could read battery status, too, it would be the ultimate... (Of course, my spreadsheet would end up being 20+ megabytes, but oh well.)
Probably the best way to tell what MPG you'd get with a Prius (or any car) on your commute, other than just renting one for a few days, would be to look up your current car's EPA ratings and compare it to what you get in real life. If you normally get 90% of the EPA ratings in your current car, then expect about 90% of the EPA ratings for whatever other car that you're looking at. http://www.fueleconomy.gov/feg/bymodel/ ... elNF.shtml has ratings back to 1985 vehicles so you can compare.