Has anyone ever tried this? It seems like a great idea for a herb garden. This is a project we were going to try this year, but I'm worried we will be stretching our scraps thin between the worm bin and the pig we've been talking about getting. In the past I've debated growing something that grows rather quick just for the compost. I'm also not sure where I would put it. So many projects, so little time. It could be a great way to get the kids more interested.
I don't know that anyone has tried it but there was a discussion about them http://www.gardenstew.com/threads/keyhole-gardens.11228/ several years ago. I just don't have room for one but they do sound like a good idea.
I went to our local community garden to see their keyhole garden. Just my opinion, but I think the soil would be too rich for herbs to do well. Herbs generally taste better and thrive in a leaner soil. Also the keyhole's bottom tier was pretty damp, which some herbs don't like.