I must admit that I was interested a motorized bicycle for a while. But then I noticed that none of them go very fast. 15 mph top speed? I can go that fast sans motor, without too much effort. What's the point?

OP: If you're going to do this, go all the way and buy a motorcycle/dirtbike (or a Vespa). Honda motorcyles, brand new, go for under $3k and for under $2k you could buy one of their scooters or dirt bikes. And you can easily find a used motorcycle for $5-600.