I've never seen one in my life. I've spoke to some of the people in the local shops and cafes near the office, and they've never seen one either. A mate of mine works in a restaurant down the road, and he says they've got a few in, but have never had to use them because no one ever uses the Bristol Pound. The only people that they've ever seen with them are tourists or foreign students that want a souvenir from their time in Bristol.
They're nothing more than a tourist novelty. If they're nowhere to be seen in Stokes Croft, probably the key independent area of Bristol, I seriously doubt they're used anywhere else.
I also heard about Portland bucks while I was in Oregon, but nobody had those unfortunately. I'd really like to get some in the future.