I thought about them a few times wanted to use them. But every time I thought "Hmm, wonder if anyone else is using them... I better stay away if I hit some bug or untested corner case".
I used POSIX message queues in production once. They worked fine, as a replacement for TCP when on the same machine, with reduced latency (roughly 75%). But eventually I switched to Unix Domain Sockets which offered similar good performance with a more familiar operating model (e.g. no worries about cleaning up old queues, and it's just another type of socket). This was on Linux 2.6.32-ish, and the POSIX MQ implementation had no bugs that I noticed over some millions of messages. Which is more than I can say of the UDP implementation.