They are getting cheaper and yes, they are great. I have a e-twow scooter. It has a 450w motor. I think that an e-bike would be better because the wheels are bigger, but for some reason there are only 250w bikes.
250 watts is the legal limit in the EU, whereas most states in the US consider e-bikes providing up to 750 watts of power (about 1hp) as still "bicycles" (meaning no registration, license requirements, insurance, etc). A lot of the retail bike manufacturers are designing motors for the 250 watt limit unfortunately, and although I have it on good authority that many bikes actually exceed their reported power limits, they're still dinky compared to DIY kits.