My mates and I went for a whole day session (8:30-4:30), ended up using around 700 paintballs, and without a doubt, I can say it was the most intense experience in my entire life.
Pinned, with legs cramping up, outflanked my a superior force, with your marker jamming up, and running low on paintballs...
*sigh*
We mostly played objective games, such as capturing a castle, a wall, flags, or bringing a "bomb" to an objective. We also did a few elimination games.
In all reality though, going without armour is the real experience. It stings like it should when you get shot.
I've personally done some incredibly stupid things, simply for the fun of it, i.e. exposing only my head out of cover (headshots aren't kills), ending in the enemy using quite a bit of ammo, and with a completely orange-d visor.
Also, take the scenario in my second sentence, except add the fact I stood up out of cover, shouted "COME GET SOME", and maniacally laughed as paintballs hit me in the chest over and over again.
tl;dr: Paintball is awesome.