You’ll probably find that you’ll be spending a lot more on gas because you’ll never want to get out of your car. 
Okay, seriously this time…
Probably the #1 thing you’ll need to figure out is whether you want the Mini Cooper or the Mini Cooper S (and/or the convertible versions of either). I’ve test-driven both, and was underwhelmed with the performance of the “regular” MC. It was the first time I’ve ever literally put the pedal to the floor, and the car wasn’t in a hurry to break 60. I have another friend who test-drove the MC with a couple family members (4 people in the car, including the salesman), and she said she had similar problems with acceleration.
So, obviously, I took the Mini Cooper S, and I couldn’t be happier.
Well, that’s not entirely true, and here’s why. About a year after I bought it, personal circumstances (i.e., tricking a girl into becoming my girlfriend) required that I move a rather significant distance, bumping my commute from 2.5 miles to about 34. Since I get about 25 MPG here in Los Angeles (where the freeways are slower than dirt during rush hour), fuel costs can be something of a pain.
In terms of the driving experience, however, I couldn’t be any happier. The engine and (more importantly, in L.A. traffic) brakes are incredibly responsive, as is the steering. Handling is also go-cart responsive, and the car itself is a beaut.
During the first 36,000 miles of ownership, maintenance is cost- and hassle-free. I took it in once so far for routine maintenance, and they didn’t try and stick me with any fees. I’m not sure how much maintenance will be after the free-maintenance period is over, but I’ll jump off that bridge when I come to it.
One issue you may run into is cargo space. If you have the luxury of folding the back seats down (which takes less than a second), you’re set. If you routinely have more than one passenger (i.e., you can’t fold the seats down), then you may be inconvenienced by the fact that the trunk, such as it is, is prohibitively small.
Actually, if you routinely have more than one passenger, you may run into problems with the back seat being relatively small. At work, I have a 4-person lunch carpool, and we take turns driving, and the people in the back seats of my car look a little cramped. That said, they haven’t complained. That said, they’re not the types to complain…so take that FWIW.
Overall, yeah, I love my car. Like I said, I have a 30+ mile, 70-90 minute commute, and it’s more than bearable in this car. Keep in mind, I’m 5’5" and generally like driving anyway, so YMMV (somewhat literally).
One more thing to mention: Depending how picky you are on what you want on/in your car, you may have a wait ahead of you, as they will custom build your car if you don’t want any of the ones on the lot. I don’t quite recall how long my car took, but I think it was about 6 weeks, and that was well short of the time they quoted me, which I believe was around 3-4 months. Not that I imagine this will influence your decision too much, but it’s something to consider.