My personal limit with a red dot would be 100 yards. That has less to do with the dot size and more to do with the lack of magnification.

I have recently been playing with the MRDS type reflex dots like the Leupold Delta Point and RMR. My initial thought was that they were not quite as good as the tube type sights, however the more time I get with them the more I think they are pretty evenly matched with the tube style.