I have a love-hate relationship with my IGH. My commute is 15 miles each way, a mix of downtown city (stops & starts) and rolling suburban hills.
loves:
+ in the city, the IGH is invaluable for the frequent and often unexpected stops. being able to shift while sitting at a stoplight and thus not having to brake while shifting while pedaling is huge fo rme
+ shifting seems smoother than my derailleur was, though it was low-end so maybe that's not a big difference
+ I used to always be tweaking my derailleur setup, but I haven't touched my IGH in the months that I've had it
+ once I fell over on my old bike, which pushed the derailleur into the spokes
hates:
- the 8 gears on the Nexus don't give enough range. heading down hills, I could use another gear or two on the top end. going up the hills, which is a more frequent problem. there just isn't a true "granny" gear, so I stand up on the really steep hills. the Alfine setup may have more range, and I'm told the Sturmey-Archer IGH has a much-lower low gear and much-higher high gear
- not sure that I have an opinion on the whole "efficiency" debate, but the direct-drive 5th gear does seem more responsive than all the others.