Having done a lot of paintball, and a whole lot of sim training (or at least being shot at in sim training, ah the joys of being the intern), I personally prefer sims to paintball in terms of pain. A sim round is barely even discernable through a ballistic vest, or a sim's protective vest and a denim shirt. Even with just a denim shirt, it won't break skin, it'll just leave a small bruise. A paintball will live a larger one, a much larger one if close.
I've been shot by an MP-5 running full auto, 3 AR-15's at once, and more pistols at once then I want to count (though since it was the whole stack I think it was 8), and sure it can sting but with proper safety it's effective.
Proper safety BTW is AT MINIMUM FULL HEAD AND NECK PROTECTION! Highly recomend long sleeves and pants, a cup or chest protector for the ladies, and gloves.
They are as noted Extremely expensive, at bulk rates it's still around $1.50 per round (that's not a type, one dollar and fifty cents). Sim fire fights get real expensive real fast.
However they have a couple of things going for them other systems don't. First is the fact you can covert YOUR personal weapon to fire sims, getting your exact trigger, draw stroke, etc. Secondly they are ballistically accurate for longer distances then airsoft or paintball (though a long barreled pump action might be more accurate). Third they are really easy clean up, the stuff is colored soap, not fish oil based, or little tiny plastic balls. Lastly the crack-pop of a sim gun while not even close to a real weapon, is still much more authentic then either sims or paintball, and man does audio add to the experience.
Just my two cents,
-Jenrick