I only have two rules about these

1. No aiming at heads, faces or glass.
2. Don't come to me to say it stings to get shot.

My boys invested some of their money in Nerf guns yesterday. I must say, on the cost-per-use scale these seem to do pretty well. The boys have spent endless hours shooting each other and a decent amount of additional time looking for lost darts and waiting for darts to fall off the ceiling. The guns are reasonably sturdy, manual-loading, and make no electronic noises. They do make a loud click when the trigger is pulled, which almost eliminates the need for the boys to make those shooting-bullet sound effects (p-shew! p-shew!) that drive me nuts.