I have a single shot 20 that I wanted to put a reflex red dot on.  It's a gun that I don't carry much and I didn't want to spend too much on a high end sight.  I saw the new Bushnell RSX100 advertised.  A $100 reflex sight.  Reviews I could find were somewhat mixed, but mostly good. Very few had it on a shotgun tho.  I found one at a local shop and bought it.  It looks pretty well made, the glass is clear and doesn't have the strong blue/green tint some do.  I like that.  The dot seems plenty crisp and bright enough for a very bright sunny day here in Texas.
I haven't even mounted it on the gun yet but I'll get some rounds thru the gun when I can and I'll let you know if a $100 reflex sight will hold up on a single shot. 
Anyone on here tried one yet?