Nice rifle! I purchased a Pumpmaster 760 and a 664sb like yours for my girlfriend and I to plink at our "shooting gallery" in the back yard.
Basically shooting into my toolshed with cardboard, then metal backstop (no ricochets).
The 760 wasn't too great- it shot all over the map. The 664sb was awesome, so I gave it to her and returned her gun.
Power is decent, but pumping it up 10 times to get the 700~ fps each time sucks. I decided to go with power/quality over quantity so I got a single shot springer instead.
Powerline 1000s
This air rifle is insanely powerful. Not as powerful as some gamos that go 1600fps, but I got it for $59 on sale at Academy (normally 79).
This thing will blast through a powdercoated steel pc case, whereas the 664sb will only leave dents in it. I put some Gamo PBA Raptors in each gun and the Powerline will break the sound barrier (loud).
Both guns are good, it just depends on what you awnt to do. I think I'm going to go back and get a fixed barrel springer liek the gamo CFX.
This sport is fun . I don't shoot live things though. I can't wait to shoot a 2L bottle full of dry ice and water... he.he.he.