I've got an SMK QB79 C02 powered rifle. I bought an adapter to allow it to take 12g CO2 bulbs rather than the larger bottles it uses in stock form. The benefit is that it is very light, and being CO2 no recoil as well as being quite quiet. c 20 - 25 shots per CO2 bulb. Good fun gun, accurate (fixed barrel) and cheap - I paid £60 (about 2 years ago). It's bolt action to load which is kinda fun too.
I also have a Gamo CFX Royal - heavy, well made and accurate (fixed barrel, underlever action), but a noisy spring and I have a diret cheap SMK break barrel - £30 new - works ok, stock was nearly orange varnish so stripped it and oiled it.
The QB79 is my favourite.