I have a Burris Fastfire II on my Mossberg 500, and I love it. It's the same sight as the SpeedBead, but mounts on a rail. My receiver was already drilled and tapped for the rail, so modifying it was a non-issue for me. It seems to me the SpeedBead would be mounted way too far to the rear of the receiver, but that is personal preference.
For those who think a red dot optic has no business on a defensive shotgun, that's your opinion, and it's worth exactly what I paid for it. The Fastfire allows me to acquire the target more quickly and accurately than a bead, with both eyes open to retain peripheral vision, while maintaining a more comfortable cheek weld. That's money well spent, in my opinion.