We use it at Mighty Spring (http://www.mightyspring.com) and it's pretty good! The documentation around backend setup is a bit confusing and doesn't cover some corner cases (like testing on dev servers) but with enough hacking you can get it to work. The front end plugin I went with (https://github.com/altryne/browserID-jQuery) needed a bit of tweaking (to both the code and docs, which was submitted to them), but other than that, relatively easy setup.
Our site is uniquely targeted at developers, so I felt that using Persona as a login option was only natural.
Our site is uniquely targeted at developers, so I felt that using Persona as a login option was only natural.