I test drove an MS3 when I was car shopping a few months ago. The engine was great, turbo spools quckly (faster than my Legacy GT), and the short gearing in the 6-speed means you can torque it up from a low RPM in a high gear since the boost comes on quick. The handling was nice, very little body roll, but if you have bad roads you might not like the ride quality. If someone gets 21 MPG in their MS3, I'd have to guess that is all city. From what I read, these things should get 25+ on the highway - which is pretty good for 260HP IMO.
MPG can't be much of a concern in any car that is actually fast. If you lay into the throttle of ANY car to enjoy the power too much, your mileage will go down the tube. A turbo car is no worse than a larger displacement non-turbo, it's just that the turbo motor generally uses less fuel when off the boost. But with the quick spooling turbo and short gearing in the MS3, you're in boost really easily.
Now what didn't I like about the MS3, why didn't I buy it? Well cost aside (I was shopping used & trying to stick to a budget), it was too 'boy racer.' The interior had all these red accents to it and the radio (A) was crappy in design+features and (B) had this gay pulsating red light on it. I fit pretty well in it (6' 4" 250lb), although putting it in reverse (R is left of 1st gear in this car) and/or using the parking brake (which is left of the center console, i.e. at my right thigh) was a little close for comfort. Also, what I really didn't like was the lack of interior amenities. No leather, heated seats, etc. I think the higher end model has HID's, but still - I wanted a step up in amenities and that wasn't it.
I ended up with a 2006 Subaru Legacy GT, tastfully modded in the looks, features, and power departments. I'm happy to share my opinions on that car if you're interested in one.