I have driven one, it was pretty uninspiring, its got to be said.
I took it home for the night on my normal commute and found that a thirty-minute journey took forty-five minutes, whilst at the same time using more fuel than my 3,0 Omega.
The technology in general is nothing too special, I think that they sacrificed a bunch of good stuff just to get to market as quickly as possible.
It's real appeal is for people who like to make a big splash about being green, without thinking about the real issues behind it. As has been pointed out, the energy used in building the vehicle is far worse than any other vehicle that I know of.
regarding the engine thing, I thought it wasn't that special - I've used that technique in engines I've developed for manufacturers before now. It's relatively easy to do using "leaky" hydraulic tappets, or "rapid leak-down lifters" as the yanks call them.
after all, the greenest car is the one you already have !