Me and the missus have used dynamos for commuting for the last 8 years - absolutely brilliant. I've used 4 different Shimano dynamos on various bikes, the DH-3R30, DH-3N80 have been fine on road, and DH-S501 fine off road. Only the XT one (DH-T670) got sad bearings after a year of off road use - I'm now seeing if the Exposure/SP dynamo fairs any better.
The more you pay for a Shimano dynamo the lower the weight, and in theory the bearings are supposed to be better, but my most expensive Shimano XT unit was the only one go bad.
For off road lighting the Exposure Revo is great, has better low speed performance than the Supernova E3 Triple (which I also had but sold on), but I still like to add a cheap Chinese helmet light for the twisty stuff. A dynamo light may well work fine on the lid as a directional light but I don't like being wired onto the bike so I haven't tried it! I did the last two Dusk 'til Dawns using only the Revo and no helmet light. Plenty of light for that sort of riding. It will dazzle on road though as the beam is symmetrical.
B and M's front lights have been good enough for our commutes. They rate their lights in units of Lux. I have the IQ Fly (40 lux) and it's fine for even towpaths, but if you want USB charging too and a pretty impressive beam, they have just released the Luxos U (90 lux) - check out following for beam shots Peter White.
I haven't tried the output port from the Revo lamp, Exposure say it won't charge USB devices via the USB boost cable, but it appears plenty of people do just that. Other third party charging solutions, that can be added to any lamp, vary from the cheap (Kemo) to the neat but expensive (The Plug II).
Apart from the Exposure dyanamo and light I have found Rosebikes fairly unbeatable for prices on dynamos and lights.