my fun car I'd rather not drive to work. I'd be a bit nervous having to leave it outside all day.

the Civic Si is plenty fun. A little fun > (a little more fun + letting people down)

 

Both of my cars are fun in their own way - with the S2000 obviously getting the nod as the more enjoyable of the two. Even in my previous 2005 Civic, I really didn't mind the lack of a fun factor the majority of the time because so much of the driving is in traffic where you can have 'fun' anyway.

 

I know two people who have owned a Cooper S and BOTH have had serious mechanical problems in the first 3 years of ownership. One of the guys is even on this THIRD engine in the car... something about oil pump failure, then a botched repair job.

Personally I'd probably look for a more reliable car myself. To each their own.

 

I almost bought a Cooper S when they first came out, but I don't like buying new designs the first year plus the dealers all had markups. I kept an eye on the forums though and read quite a few problems that were common to owners. I would've gotten one in the following year or 2 when they resolved those initial problems, but I ended up with an RSX-S instead. The Cooper was much more fun to drive though and I liked the low end torque. The RSX-S had a great engine and tranny, just hated the suspension. I drove a coworkers MCS now and then and every time I did, I kept falling in love with the handling. Great little car.
A few years after that, I almost bought an original Austin Mini after testing driving a bunch of variations...from pickup to station wagon, left and right hand drives. But too much exhaust fumes in the cabin.
Like their parents (BMW), they won't be as reliable as a Honda and you'll have a large share of owners with no problems and another large share with problems. As long as you're ok with that, it's an awesome car.