Most have covered the main points here. If you live someplace sunny like I do, it just makes a 'vert that much better. I honestly have the top down about 65% of the time I drive the car. It can get a bit windy at highway speed with the top down. It's not enough to bother me, but it may bother some people. IF it does, just keep an eye out for a factory windblocker. There's typically one to be found on eBay and they end up going for about $250 or so.
Every once in a while I'll hear a little squeak here and there, but that's rare. One guy either here on on m3forums has a nice DIY on how to lubricate everything on the top that should make it even quieter than it is now, I just haven't had a chance to do it yet.
While it's slower than the coupe by a tick, it's still very fast.
The top is very easy to use. 1 button up or down. It seems to work well. It's not bad to drive with it up. There's a nice inner liner so it looks nice and smooth. It's reasonably quiet, I'm sure it's not as quiet as a coupe, but not bothersome at all. One bonus is it's quicker to clean\wax the car with no paint above the windowline. You WILL have to maintain the top though, but not every wash. Many (me included) really like the Raggtopp cleaner and sealer. It'll make water bead up on your top almost like wax. You'll want to ensure you get a car with good leather, and stay on top of it more than a coupe, as UV\Sun is the enemy.
If you're every going to run anything from the trunk to the interior (stereo cables, etc), it's a bit more complicated, as the rear headrests are part of an integrated, pop up roll bar system. This is also the reason I can't do a stealth Valentine 1 install in the rear headrest (I may try the front, but haven't gotten to it yet).
I used to live in NJ, and while I wouldn't be able to drop the top nearly as much as I do in Florida and I'd still go for it. I just may invest in a hardtop for the snowy months.
This is the most overall fun car I've ever owned.