I have used those style stocks on a mossberg 500 (friend owns one). I have to say that they officially suck, for two reasons:
1. The action lock lever (pump/slide release button) is almost impossible to press with your hand on the pistolgrip, unlike on the regular rifle-style grip (use your middle finger). It goes from completely natural and intuitive to completely awkward. Also, the tang safety is now out of position for use by the thumb. In other words, to operate the weapon, you have to take your hand off the pistol grip. Useless for any high-stress situation. It's kind of like putting the stick shift in your car behind the driver side headrest. You CAN reach it, but do you want to do it in a high-stress situation where split-second decisions are a factor? On an 870, the controls might work better with a pistolgrip. For now, I'll take the 500 with a rifle-type grip.
2. Those stocks are really light. It's fine if you are only going to put low-brass 7/8oz birdshot shells through it. However, when you combine the low weight of the mossberg 500 frame with an untra-light stock and any serious HD load (slugs & buckshot), particularly 3" shells, your shoulder will not thank you, even with those recoil reducing stocks.
So, to sum up, those grips are not ergonomic, and make running the weapon through training drills or actual combat a nightmare, and the lower weight makes recoil a bitch.