As a Pyro main, here's what I have to say.
First of all, in lower level's of 6v6, almost anything is viable. Including full time Pyro. In higher divisions and playing with highly competent people, it's not. Pyro may seem like an over powered class or w/e, but it's not. People complain about it being annoying but it's simple to kill a Pyro if you know how to, and if you know how to, you are probably playing in higher divisions. It's easy to fight a Pyro if you know how to.
Pyro's really should ONLY BE USED for last points, and neutralizing damage output and ubers from the other team. In lower divisions, it's not that bad to have a nice pocket Pyro to help bombers not able to get to you.