I picked one up a few months ago, and through a variety of circumstances I didn't get around to trying it out until this morning. For comparison purposes, I also own a couple of Remington 870's in various trims, and an old 26" Mossberg bolt action shot gun that my dad bought out of an Eaton's catalogue in the early 50's. I've also tried quite a few other shotguns over the years, including a buddy's Beretta 1201.
After running more than 30 2-3/4" 1oz slugs through my M4 today, I'm thinking of selling my other shotguns.
Loading it is a breeze, with easy, low effort insertion of the shells. Occasionally, other shotguns I've loaded have had sharp edged receivers that can cut or snag your thumb as you insert the shells. More commonly, my 870's take some serious shoving to get the shells into the tube, and as the shells travel into the tube, my thumb detects little snags. The Benelli only requires light pressure on the shells to load them, and as they are being inserted I didn't detect any snags or binding(No need to wiggle my thumb as I'm pushing the shells in). This translates to much quicker loading time for me.
At 25 yards, my buddy got a couple of bulls eyes when he tried out my M4 and I managed palm sized groupings. That's much better accuracy than I get with my other shotguns, but to be fair none of my other shotguns have the terrific sites that the M4 has.
But, the one thing that surprised and impressed me more than any other thing on my M4 was the trigger. It has a very short reset and a strong, tactile feedback when it resets. While not equal, it reminded me of the trigger reset on my CZ Shadow or some of the other IPSC handguns that I've tried before. A trigger like that, on a shotgun, was a very pleasant and unexpected surprise.
For the fun of it, I ran some carefully aimed rapid fire. (ie. Doing my best to reacquire the target as soon as possible and quickly dropping another hunk of lead on target, as opposed to just barfing lead downrange and making a lot of noise.) With my pump guns, I've never been able to get close to the speed and accuracy that I got out of my M4 today.
I'm sure there are fellows out there who may be able to make a pump perform better than I can make my M4 perform, but for me it's no contest: In my hands the M4 performs better. If you have some extra cash floating around, buy one.
