Well, it is blazing fast, very light in weight, and has nearly no kick. I refer to my Fastback cocker with PBX frame and MQvalve... so far as I know, I am the only one outside of PBX with a working frame. I will post pictures of this awesome beast after I get done anodizing it. Efficiency, I am not yet sure of, but I was told to expect slightly better than average cocker efficiency with the MQ. My other Fasty (Eblade 2) has a regular valve (for now). It is nice and quick, and has no problem whatsoever with 15 bps PSP mode. Theoretically, it could cycle a bit faster, but I can't shoot that fast for a whole second.
As for "should you get one;" I'd say yes, if you can, if you like cockers and can work on them. Rob Long (Turtle) and Doc Nickel (Fastback) are hard to get bodies from. Doc, I hear, is out of stock. His Fastback bodies are beautiful, though. I don't know anyone with the Fireball Mountain body, but I trust them as a seller before any of the other makers of halfblock cockers. WGP is releasing a new Black Magic sometime soon that is basically a halfblock, although they left most of the upper receiver in place, sort of like a Catapult or PBX cocker. There is a thread in AC&Q about the new Black magic, incidentally.