Yep, I've been there too. I was drafting traffic in primary at ~30mph when I was suprisingly undertaken by a chap on a MTB in the cycle lane. It took me a while to work out what was happening, he was doing 40mph easily. I stopped to chat with the fella at the lights. Having recently done my research I asked him if it was one of the 1000w motors, to which he smugly responded "Yes." Then as he darted off on the green light, he was nearly splatted at the next junction, a car that was pulling out clearly hadn't expected him to accelerate so quickly from the lights. In the wrong hands, no doubt a few people will be hurt with those high powered motors.
I'd be interested in trying one if I had a longer commute, but then realised I didn't want to get points on my licence or be done for driving an uninsured/illegal motor without appropriate lighting and number plates!