We purchased our 2007 Prius on March 31st of 2007, and it rolled over 10,000 miles sometime during the last weekend of December on a trip to Memphis (trip avg. 52.1 MPG, 95% highway driving).
We have been getting a fairly consistent overall average of about 50 MPG, a little less in the summer (~48 MPG) with the AC running (or when the leadfoot Mrs. is driving) and a little more in mild weather. By my calculations, we have saved approx. $700 in fuel costs.
The only other cost has been the regular 5000 mile service, which was about $90. The only problem we've had (knock on wood) was a power door lock that jammed. Toyota replaced it under warranty at no charge.
So far we love it. Toyota sold 181,221 Prius hybrids in 2007, up 69% over 2006.