I had one on my 1st M4A1...
They're wicked, and I've gotten lots of kills with it up to 4/5 people at once at times. The only downside I found is it made the M4 Really front heavy and after a full day of game my back would be a little sore.
None the less if you have the budget then get one for sure, they're hella fun
I'd recommend getting 4 to 6 cells for it as well and I know you can buy pouches that hold them somewhere.