The slide fire for the AR-15, you mean?
I've seen and handled one, haven't used one.
Seems like it breaks down something like this...
Advantages:
- Eases the "bump fire" process considerably
Disadvantages:
- Expensive
- Must be removed and replaced if you ever want to shoot the thing normally
- Doesn't aid in accuracy
- Doesn't make the ammo any cheaper
If you like bump firing an AR for kicks and giggles, it's probably a good solution. For anything other than turning ammo into noise quickly, it's pointless.