I've got the Revo/SP setup on my "road" bike
DSC_0135 by ir_bandito, on Flickr
Yes, it gets duller at slow speed, but even at the slowest, about-to-fall-off speed its fine. And the extra-bright mode kicks in at suprisingly not very fast at all. then its good for 40mph+ descents in pitch black. That is on road though.
If anything, my biggest complaint is that its too bright, especially on road. I'd like to be able to switch off the extra 2 LEDs when riding at speed in a lit, built-up area to avoid dazzling other road users.
For MTB use, it would be nice if the standlight could have an external switch, so for a slow speed techy descent (or climb) say, the light could be boosted from the capacitor.
To be honest though, if you're off road you've probably got a battery powered helmet light too, so that would probably give you enough light for the slow techy stuff, then the faster stuff the Revo would kick in.
As for the drag issue, I was amazed how stiff the hub felt trying to turn it on its own when no built up. Once built into a wheel, you can feel each of the poles contacting as you spin the wheel very slowly by hand, but as soon as its up to a resonable speed, you can't feel any drag. Unlike my old Shimano B&M setup where the light would flicker like a strobe at slow speed and you could feel (and hear) as you rode slowly.
Edit - For the record, my Rohloff with fresh oil in has naff all drag too....