Keep in mind that the problems we're having aren't with XMPP, per se, just with ejabberd. Ejabberd worked flawlessly for several months until it started exhibiting some very strange behavior and performance characteristics. We upgraded to a newer version and we suddenly ran into another, different set of strange behaviors.

That said, word on the street is that ejabberd is the best implementation of an XMPP server. AMQP supposedly offers better reliability (with RabbitMQ) as well as a whole lot of pretty cool features that Jabber really doesn't.