C.A.P.I. makes some fantastic equipment, and this is one of the most useful Preamps around for the 500 series. The input gain does the API thing, the output fader allows for a wide range of gain staging options without hurting anything further down the chain. The low cut options are fantastically useful. Really, give me a desk filled with these and I'd take it over most anything out there.
In sessions at Bear Cave Studios, it usually ends up on guitars or snare drum and never fails to deliver.
Fun fact - this is another aquisition through Gearslutz classifieds, the seller being Ville Valo of Him who turned out to be the genuinly nicest seller of gear on the internet you could imagine.