I've put one on all my AR's. Supposedly that keeps you from having to register them as assault weapons. I have a couple of lowers I haven't done anything with yet also and put them on those too. At least this saves me from having to buy bullet buttons for them. The hard part is taking the bullet buttons off the old ones I have.
They work extremely well for what they are. They are much better, functionally, than a bullet button. They do protrude from the side of the receiver a bit. Allow for an easy release of the mag once the action is opened. Rather easy to install, completely reversible if you get out of a restricted state.