You should be aware that you won't get the EPA mileage listed on the window sticker. At best, 10-20% lower MPG from your average new car, hybrid or not. Since the Prius, Civic hybrid, and Insight all great fuel misers, this could seem like a lot. For example, a Honda Insight is EPA rated to 66 MPG, but you might only see 45 MPG out in real-world driving.
That said, you will save money on gas if you drive the car far enough, usually at least 75,000 miles, or about 5 years depending on the car and how much you drive. I'm not convinced by environmental reasons for getting a hybrid. The best way to cut oil use, reduce air pollution, and save money on gas is to simply drive less.