I have a Dh 3n80 hub. At the time I bought it, it was the most efficient hub shimano did, and significantly more efficient and light than the nexus one at that time (two years ago). It’s worth checking out which is the current most efficient shimano dynamo.
It is great – I’ve run it enough to need to replace the rim and never had a problem. I’d not have a commuter bike without one – incredibly useful just always having light. I have b+m lights with light sensors, so they just turn on when it gets dark. No need to charge batteries, or take lights on and off the bike, and I never have a problem in summer when I unexpectedly stay out late.
Joe