It was very easy to pass the tests the W3C working group used to verify that they had two working implementations of the data model and protocol. Most of the test default to passing if the specified tag is not present. Basically, it's not clear whether a serious, real attempt to use this has been made. I'm unconvinced that the specification is robust enough to be useful without ending up with a lot of vendor lock-in.
The toy extension was playing around with using these annotations to alert publishers and potentially other users of typos in their articles and pages. It would be nice to have a side channel to report typos other than just using the comment section or trying to find an email address. Will the "meta web" ever catch on?
I never published it but I still might add a page about my experience on my website. I have posted about the idea there before.