In reply to mike123:
I have had a Shimano Alfine 8 as my commuter for over 10 years. I take it to my bike shop for an annual dip in an oil bath and to tension the chain (which gets done by playing around with an eccentric bottom bracket.) It has been 100% percent reliable and I live in Sheffield which for a city, has some pretty big hills and it copes absolutely fine. It is maintenance-free apart from that. Disadvantages would be:
1. It is heavy, compared to a more conventional set of sprockets.
2. I am convinced it is an inefficient converter of effort into forward motion. Maybe I am just old and unfit but everybody seems to be able to overtake me.
3. Instead of a quick-release skewer to release the rear wheel, there are two chrome nuts, so I need to carry a full-size spanner with me, in case of a puncture.
Overall, I think it is a finely balanced thing whether I would buy another one or just go for a derailleur and cassette, especially now that you can get such a large range of gears that way.