We use one for both of our dogs (border collie mix and then a basset). We kind of have to, though, since we aren't legally allowed to have a fence in our neighborhood (dumb rule). We've got like an acre of yard with all sorts of trees and obstacles so using a rope or anything like that wouldn't work, either.
Sadie used to get out occasionally when she was younger, if she got super focused on something and hit it at a dead run. That was only really during her demon days, though. It did take a while for her to understand that the little beep meant that she needed to stop...she got a shock quite a few times when she was younger.Now that she's grown up and chilled out a little, it works fantastic (half the time I forget to even put the collar on her, haha. She just knows where the boundaries are).
It works great for Babs...I don't think she's EVER gotten out of the yard. She hears the beep and backs off.
We'll leave them out there for a few hours (as long as they're not at the door asking to come in) but there's always someone nearby/on the ground floor to make sure they don't get out. I still wouldn't trust them out there completely unsupervised...we have a lot of deer and turkeys come through our yard and I wouldn't put it past Sadie to chase them right out of the yard.