For the level of performance the Si offers, the mileage one can acheive is superb. I average a bit under 30mpg, and have seen as high as 36 on slow highway trips. Not as good as a Corolla, but a hell of a lot faster and more fun!
Perhaps the second best handling FWD economy car next to the Mazdaspeed 3. Very sharp steering wheel response, only minimal understeer, and it can be pushed through corners at astonishing speed compared to the regular Civic or other similar cars.
Honda has built a name for reliability, and the statistics on this site back it up. Personally, with about 40k miles, I have yet to have a non wear-part related issue.
For an economy car, the Si is fast. 0-60 in about 6.5 seconds is plenty to get you in to trouble before you realize it. When you hit 5800RPM (VTEC), it feels like 1000lbs were just sheared off of the car. When spooled up, the engine makes you :D