P-90's probably became popular through films (eg Equilibrium) and games (eg CS, Goldeneye), as well as Stargate SG-1. Personally, I came to love them through SG-1.
I got mine a couple of years ago. It was already upgraded and modified to accept the J-style 9v batteries and could chuck 0.2g BB's out at a respectable 350fps and around 800 rounds a minute. I got it for £200, with two hi-cap magazines, and one lo-cap, a basic sling and a mag pouch. I had to buy a new J battery (the mini that came with it died) It's also a proper Tokyo Marui model, not some cheap no-name brand.
It's fully auto with a safe/single/auto fire selecter switch. As in the real P90, it has a select fire trigger too - on auto setting, pull the trigger back a little way to fire a single shot, pull it all the way back to go full auto.
"Whats that for?" "Thats the machine that goes 'ping'" PING!
Q. How many testers does it take to change a light bulb?
A. We just noticed the room was dark; we don't actually fix the problems.
MCMF forever.