I own a speed bead.
The mount for the 870 is terrible. You cannot use it with a synthetic stock -- it has to be used with a stock with some 'give.' Unlike the Mossberg mount pictured by most vendors (a big steel donut, which is probably stable enough) the 870 mount is two thin wires with small flanges that dip between the stock and the receiver. The sight itself sits on a small platform insulated from the receiver by a foam cushion, and in my experience the mount does not allow the dot to hold a zero worth a darn. I tried sighting it in at 25y with slugs, but the spread was 6" left, then 8" up, then right ... On the fourth shot, when I realized no matter how well I zeroed the sight the process of zeroing changed the position of the mount, I gave up rather than destroy the target holder.
I recently purchased a picatinny mount for the fastfire dot. In that configuration, it's a beautiful red dot. My complaint is entirely with the 870 speedbead mount itself.
Oh, and as an additional kick in the junk, when you order the speedbead you get the fast fire model 1 -- whereas the fast fire model 2 is out everywhere else, and is waterproof.