I've driven one. I was considering building a GTM so I found a guy local to me that built one to have a look. All I can say is with Ferrari and Lambo prices the way they are right now, one would be crazy to spend 1,000 hours building a rattle trap that doesn't come close in fit and finish and still costs almost as much. Is it fast in a straight line? Yes. But to get the performance they advertise you are gonna spend a LOT of money on something that isn't worth much in the end. Ferrari 360's can be had for less that $80k now, and having owned one I can tell you they are a much better buy. Also, with Lamborghini Superleggera prices in the $140's they are an amazing buy too. There's a lot to be said for a 4 wheel drive car that breaks the tires loose in second gear, lol. They are just plain fun to drive.
As far as kit cars go, the Ultima GTR would do it for me.