Done it before, it's just like having rice in your stew, it's nice but I personally prefer it with ham hock rather than beef. The sweetness from the barley isn't overpowering either.
Sounds like it might work quite well. I have used it in my chicken casserole recipe today, as I know it works well with chicken but wasn't sure about beef. I have a stew that I am going to make next week so I think I might try some pearl barley then. It is cheap (55p per bag) and you don't really need to use a lot!