I had a Schmidt XSON dynohub fitted to my Brompton. The dynamo itself is excellent. My main problems have been with wiring up the lamps and the front lamp bracket, but these are largely problems to do with the design of the bike. The back lamp used to cut out a lot, but this was due to it not being earthed properly. The wires to the back lamp sometimes got disconnected, but this was mostly due to them being tugged while folding the bikes. Hopefully, a zip tie will sort out that problem. I used to have problems with the front lamp and lamp bracket. It used to stop me using my front pannier, and I had to swap over my lamp for one of the circular Busch & Muller lamps. As I said, the dynamo itself is excellent. I don't notice any drag. I've used bottle dynamos in the past, and these were never satisfactory. Any bulges in the tyre used to cause the lamp to flicker, and the bulbs were constantly blowing.