I have the Shimano 3N72 laced up to a 26" Mavic XM317 rim, on my old Trek 7000 mtb. It works great as a dynamo, but I'd say that it is a little heavy and I wouldn't put it on a road bike. You really can't feel it when the light is on vs. off, but I'd bet I'd feel a lighter non-dynamo wheel.

There are a few versions of the Shimano, and then there is also Shutter Precision, Sanyo, Schmidt and a few others. Shutter Precision is pretty light compared to my Shimano, and I almost went with that one instead and would try that if I ever needed another.