I will echo the two sentiments repeatedly expressed: 1) People doing laser fights at the top of the stage are annoying. 2) People walking around using lasers to enhance the mood and trip out fellow audience members were welcome.
It's sort of like the glow stick stuff. It's fun when the glowsticks are the right size and thrown at appropriate times. It's not fun when the glow sticks are bigger and thrown like fastballs straight through the air.
I brought along my green laser. It's a 20mW model, so it's friggin bright. However, while it was a hit with the crowds, I kept it off the stage. Personally, I liked seeing the beams dancing around the sky above the crowds rather than watching some jerkwad bouncing it to the beat above the main stage.
I should have added that most, but not all, folks who use lasers at shows are incredibly DISRESPECTFUL of the artists and the audience.
It's only disrespectful if they shine it on the bands or on the audience, which I didn't see once. I really don't see how a couple green dots above a stage is that big a deal. I personally got a kick out of watching laser beams shoot out over the crowds. Also enjoyed the kaleidoscope patterns people would do on the roofs of the tents