Anyone using the slip on rubber cover for the butt of collapsable stock. Do they help?

yeah i use it.... it helps keep the stock from sliding from your shoulder.  especially in the winter whne you have the weatherproof jackets on.

I have one from Bushmaster, it does help shoulder the weapon better.

Any links for one of them there rubber pads. (wasn't about to open the door to the wolves on this site by asking where I could find a butt rubber)

Yup. I like it, it's good.
AB

I had one and didn't like it. I took it off. I found that it grabbed my shoulder too well! It made it difficult to maneuver the weapon between shooting positions. Especially when performing dominant/support side or cross shoulder transitions. It also tended to snag on everything when slung. On or off it made no difference in accuracy. An unecessary piece of gingerbread, now relegated to my "stuff I don't use anymore" box.

Another way to say this is if most of your shooting consists of just standing there banging away at the 100 yard line, I suppose it doesn't matter, might even help. If you are lucky enough to be able to do some running and jumping and sneaking and peeking, don't bother with it.