So, first off, fitment is a little sketch. The heat shield is really the issue here, but with some basic modification, it can be made to fit well enough. It definitely dresses up the engine bay, and makes the engine sound better, (to my ears, anyway)
As far as function, it does what you would expect. Using the subaruEdit software to log MAF before and after, the HPS does indeed allow more air into the engine, and also makes the throttle more responsive.
Does it add power? Maybe a little bit, but don't expect crazy results... it's just an intake. If you're really looking for whatever gains you can get... get a proper tune after install.
All in all though, I do recommend this for the Fb25 engine.