I've used them a few times. As you've seen you don't get a whole lot for them but you're probably not going to get any more on ebay so if you've got a stack to get rid of its better than dumping them. If you've lucky every now and then you randomly get something that's worth a fiver.
I've used them twice, but the second time they seemed to 'lose' a number of items that were in the exact same heavily-taped box as the stuff they processed.