I replaced the stock tires with Firestone Firehawks, and was so dissatisfied with them that I replaced them with Bridgestone Ecopias the other day. With the older Firehawks, I averaged 40mpg on my Prius (hence the reason they had to go) and excessive road noise on the highway route I take to work. When I talked on the bluetooth phone with the windows up, people would ask me to roll up the windows up!
The Bridgestone Ecopia's are head and shoulders above the Firehawks in my opinion. Although they are not as grippy or as responsive (they are not sports tires), they create MUCH less drag and seem to have much better gas mileage--although it is early to calculate a tank, the onboard computer estimates 50mpg vs. my last tank had a computer estimate of 45mpg with the Firestone tires (actually my last calculation upon fillup was more like 42mpg--yes, the onboard computer's calculation is off a bit--which was pretty high for those tires).
The biggest difference, however, is the road noise. The Ecopia's hardly make any more noise on the highway than they do at 5mph! They also create much less drag; I can coast along on level ground on solely electric power at 45mph (vs. 25-30 on the Firehawks), without the mechanical engine kicking on.