Not me. It's been around quite a while and all I have seen are the same videos of him pumping air through one, absolutely nothing to support his power and efficiency claims.
There might be something in the effective reduction of reciprocating mass but I don't see (as someone else posted) a robust means of transferring power to the shaft at any respectable output level, especially if used as a Diesel. You don't have to be a ME to envision some tremendous forces at play in whatever is connecting the pistons to the shaft.
Everything is a compromise and I don't see the advantage outweighing the mechanical issues. And besides - shaving weight and reciprocating mass doesn't necessarily turn into increased efficiency (turbine, Wankel).
It makes an interesting air pump though.
My curiosity is “how’s he going to make money with it”? If I could figure that out; I’d push my perpetual motion machine onto the unsuspecting dolts that believe this crap. Hey, where’s Billy Mays when I need him?