First off, I am not much of a shotgun enthusiast however I have fired my fair share of different models and different brands to realize that the M4 is a great Shotgun. Just like the M1 Garand, the design of the weapon just feels right. Some decent engineering went into its design.
I agree with ChuchSwagger (above). Get a 7 rnd tube and some ghost ring night sights (or a red dot) and its a blast.
Things I love about the weapon system: I can shoot slugs out of mine and consistently hit targets at 100 yrds, the cycle rate is phenomenal, and the recoil is negligible.
Some downsides: 922(r) is an issue to think about if you want a collapsable stock plus some other combined features, but there are a number of companies that make “upgrades” to counter the foreign part issue. The problem is that those companies are usually out of parts so getting your M4 to where you want it can take time. Some of the controls are small, Bolt release and charging bolt could be beefed up (which there are aftermarket parts for), and the finish is meh…If only I could get a tenifer coating (like glock) or take the plunge and cerekote it, I would be completely happy.
Overall, I would say if you enjoy shooting shotguns you should take the plunge. The price for the benelli is close to medium end AR15s these days. Honestly you are getting a well designed, robust shotgun. Now if you are worried about using various shells the Benelli M3 is also a great weapon ; ) Good luck in your decision.