BB as if in Ball Ballistic, not Ball Bearing. Ammo is so much better than paintball ammo, you pay what...£10 for about 1000 paintballs, right?
You don't re-use paintballs, for obvious reasons.
It's £8 for 5000 0.2g bbs which you don't re-use (otherwise you'll f**k your gun, to put it simply).
Airsoft kit is loads cheaper than paintball, you don't get covered in paint, you can shoot massive distances (in comparison to paintball) accurately and of course the ammo is masses cheaper and easy to reload.