I'm using MQTT on my current project that involves interactive applications running on solar powered devices.

I'm using RabbitMQ for the broker, it supports MQTT too, this means I can use MQTT on the clients and the services on the backend only needs to understand AMQP.

I made a simple RPC system on top of it, unfortunately it's a homegrown solution as I couldn't use RabbitMQ RPC features with MQTT.