Air gun question

I'm looking into getting a cheap air gun, maybe a pistol. I just want something I can piddle around with. I have a Benjamin but I'm considering a CO2 this time, but I have zero experience with them.
Do any of you know anything about them?

Questions I have

1) how many shots can you get from a cartridge
2) does it get progressively weaker or is it measured burst that are consistent until empty
3) is it worth it to get refillable cartridges and all the connections needed to refill


Thanks in advance

I like my .22 pistol with rat shot. ;)

LJ can prolly help you with the air gun ... he plays air guitar. :rockon