I don't think of the Mini as a great fuel-saver at all. The "S" gets 25/32 MPG (city/hwy), but requires premium. It does make 168 HP though, which doesn't make this tiny car's mileage look so bad.
The regular version also calls for premium fuel, only makes 115 HP, and only gets 28/36 MPG. That doesn't seem like the best deal if you're looking to save gas. By contrast, a Civic EX makes 127 HP on regular fuel, and gets 32/37 MPG.
I realize that most people don't buy these cars for their mileage, but those who do, especially those who buy the base model, are in for a somewhat sour surprise. People looking to save gas should concentrate on the Civic Hybrid sedan, Prius Hybrid, and Civic HX 2-door. (Fuel-frugal folks who don't mind the fact that diesels still pollute a lot more than gasoline engines might also consider the Golf and Jetta TDIs.) Other people looking at Minis should just step up to the "S" version, because a "big" engine in a small car is what the new Mini is really all about.