MILES (Multiple Integrated Laser Engagement System) is definitly a POS. I'd recommend paintball or Airsoft for backyard fun. Another, more expensive, option is Simunitions.
MILES tends to work when it wants to, it's as if it has a mind of its' own. If you bump your harness on something, you're liable to set it off. Also, the laser beam is easily blocked by the thinnest of concealment, making bushes into instant cover. It's also very easy to cheat with MILES. Take the battery out, and you'll live forever, or until you put the battery back in at least. Even then, all you need to do is rekey it to shut off the (annoying) beeping. The keys are easy to replicate too, so people can cheat even if you have the only key. Another way to cheat is to put 100mph tape over the sensors. Of course, one cool thing about MILES is the ability to make silent shots. All you need to do is tap the emitter in the right spot to fire it, without the need to shoot the weapon. A bit tough to aim this way, but do-able.
Just my opinions, garnered from a lot of experience using MILES.
Frank