I just built up a Salsa Vaya and we put a Schmidt SON 28 disc in the front wheel. Other than the $300+ cost, I am absolutely blown away by this thing. Try as I might, I can not detect a difference with lights on or off. I am running Busch and Muller Lumotec IQ CYO R Senso Plus in the front and Toplight Line plus in the back -- also fantastic and amazing lights.
If you turn the axle with the wheel out of the fork you notice notchiness and wonder what you just spent $300 on. Drop it into the fork, though, and it rolls and rolls. Peter White (I think) describes this as 30 pulls and pushes that almost cancel out. Turn on the light with the wheel spinning, and it noticeably slows.
Like I said though, riding the bike, I don't see how you can ever tell. No noise or vibration at all ... just lights you never have to worry about!