I deleted my first post which was "Yes, they really are legal, and Yes, they really suck, don't waste your money". Then I saw that you had already bought one and didn't want to flame you.
I went on to say, save your money and learn to bump fire, at least that's free. All you need is a finger, the thumb next to it, and maybe a belt loop. The theory of operation is the same. With your offhand, hold the rifle forward of the trigger, like the forend/handguard. With your firing hand, put the thumb through a belt loop on the side your are holding the rifle, which you hold under your arm, and the finger on that hand on the trigger. Pull forward with your offhand while keeping your triggerfinger held back by your thumb/belt loop. The rifle will fire, and the recoil will push the rifle backwards. Keeping forward pressure with the offhand will pull the rifle forward again, causing you to pull the trigger. If you can find the sweet spot, you will can produce a continuous fire as the rifle rocks back and forth. It's just like many of the trigger systems except instead of a spring or what not, you are simply using your hand anchored to yourself. Do a thread search on bump firing and you will probably get a better explanation. The problem is, like many of the trigger systems, is that you cannot grip/hold your rifle with the firing hand, it must be held and pushed forward with the other. Some rifles will be too heavy, unbalanced, or you may not be able to grip and hold it adequately, especially while it fires continuously. Depends on the shooter, and it's not very safe if you loose the forward grip cause if it falls, the weight of the rifle will cause the finger still on the trigger, to fire while it swings down or wherever.
If you are quick with your finger, and have a good trigger, you should be able to fire quickly enough to waste your ammo, just like real full auto. Full auto is only good for cover fire, room clearing, and entertainment, IMO, unless you have a really, really good muzzlebrake.