I did it a few years back, and the biggest problem is the honor system. Especially if you have a long range shootout (range is very good on the guns). The range takes the sting out and a little hard to see, they can be hit and just carry on and pretend it didn't hit. You cant deny I paint splat.
The setup was an old factory they had access to with various levels, stair ways, open areas with just concrete pillars and some smaller rooms, also a rainy day so was pretty dark. Ideal conditions and setup, and it was a bit of fun, but at the end of the day when it all finished up and you see the guys that have brought heaps of gear and are dressed like spec ops guys, you cant help but feel like a bit of a dick with play guns and playing dead being hit by a plastic pellet (or not if your a cheater).