i built one last summer.. on mine you can pedal assit. i went about building it from parts so i used 4x12v 7ah lead acid bats series with a phoenix brute (for the most torque) rear wheel w a crystallite 48v controller on a diamond back mt bike with full suspension.
here is the site i got the stuff from ..
http://www.electricrider.com/crystalyte/phoenix.htm
it was cool at first but the novelty wore off real quick... im assuming it does about 25mph per the specs. but the charge takes to long. the bike weights 50+ lbs, its never fast enough, and to make what i really want to do go faster weigh less and run longer... you would need a controller to handle lipo and sink mass bank into a battery system and put it up on like 60+ volts.
thats my 2 cnts
ill try to get pic up but the bike is now in the back of the garage for winter
overall i prolly could have spent 2k and got a dirtbike