They look like lots of fun, but I'd rather have an Ariel Atom, a street-legal stripped back hot rod.
The model demonstrated here
http://www.youtube.com/watch?v=WaWoo82zNUA weighs 500 kilograms (1100 lbs) and produces 300 brake horsepower and does 0-60mph (100kph) in less than 3 seconds, and was the second fastest car top gear have yet tested. I've read of a 500 horsepower model.
Test driver: "It feels like my brains are being sucked out..."