Its pretty straightforward n simple to setup and use.
For testing purpose - test.mosquitto.org is best.
Also using an online broker is best because then no port forwarding is required to connect over internet.
Also it is secure unlike port forwarding
I already have a program running right now - where the esp is uploading temperature data.
This data i m reading on my android phone over 3g - using myMQTT app.
The code for the program i have used from a tutorial (I dnt have much programming knowledge myself)