I have SMG on mine and I'm very pleased with it. I had a Smart Roadster before my Z4, so I knew what a bad semi-manual was like and what I'd be letting myself in for.
In terms of maintenance; as far as I'm aware there isn't any cost difference in routine maintenance between the transmissions (except perhaps gearbox oil, which may cost more for the automatic). Contrary to popular belief, the Z4's SMG is actually fundamentally the same gearbox as the manual with the addition of an electro-hydraulic actuation system (see page 15 of
http://www.unofficialbmw.com/images/05 E85 Driveline.pdf). I'd expect it to wear the clutch a little quicker than a manual as it'll slip it more that you'd want if your not attentive at junctions, and Sport mode changes can be pretty brutal. And obviously extra components mean more things to go wrong than a manual (the pump failed on mine at three years old), but the same is true for an automatic.
As for driving; I think the key message is that even though it has an automatic shift mode, it isn't an automatic. If you drive it like you would a manual it's fine (i.e. you wouldn't keep the accelerator floored changing gear in a manual), but you can't drive around without any regard for gearchanges and expect them to be seemless all the time (although they mostly are). Not sure what problem you mean when you refer to driving up hills. It'll drive slow enough for me and keep it in gear (900revs in first). I think the only feature worthy of note is when setting off; it lets out the clutch a little sooner than you would in a manual, so unless you use constant gentle throttle (or alternatively totally brutal throttle!) you're at risk of kangarooing it until you're used to the car.
Oh, and you get launch control with SMG. Although I've been too scared of lunching the clutch to use it yet...
with me