Firstly I'd agree with several of the other reviews here (and this goes for any of the gun attachments, not just the Sony offering) that the viability of this attachment entirely depends on the game being played. I found the demo for Time Crisis difficult to play due to the need to use buttons on the top of the move controller during the game. However when using this to play Dead Space Extraction I found it excellent. The navigation controller handles all of the secondary functions leaving the Move controller as a gun alone meaning no requirement to tinker with buttons on the top of the 'gun' while playing.
Hardware wise this is a fantastically well made piece of kit. The Move controller fits snugly into the gun body with no detectable movement once in place. The grip is comfortable and the weight good. It also has the distinct advantage of looking like something out of a 50's B movie when in use.
I thoroughly recommend this if you're planning on buying a gun attachment and can't wait to play Killzone with it.