Airsoft is great fun, but definitely not cheap
I own a Tokyo Marui P90 myself, and used to play with friends when I lived in Japan.
I had a Desert Eagle as well (the 3,000y electric one, not the 15,000y gas powered one), but left it behind when I came back to America. I wish I had an opportunity to play again, but because I am so busy, nowadays it is basically a $250 decoration. If you've got the cash to burn, and can play enough to get your money's worth, go for it.
Archaix, they are in fact called BBs; airsoft guns typically fire 6mm plastic BBs, and you can get them in different weights (lighter ones are faster, heavier ones more accurate).
How much they hurt really depends on your weapon. The weakest are probably battery-powered (ie. AAA) handguns, which won't be felt at even a modest distance, but can still break the skin at close range. More powerful are battery-powered sub machineguns and air cocking handguns and rifles (where you manually pull the slide for each shot). These can sting a bit. My P90 will do a number on fruit and go clean through cardboard boxes. Gas guns are the strongest, and can do some serious damage. I had the chance to play with a gas-powered AUG, and it shredded whatever I put in front of it. Nobody wanted to be on the receiving end of that gun
However, even this will still pale in comparison to the pain a paintball gun can dish out, and if you use cheap handguns or wear thick clothing there will be times that you won't even notice getting hit.
Also, Chony, I hear that biodegradable BBs aren't as strong as plastic ones and break easily; maybe that is why they don't seem to hurt very much?