Well, I'll actually answer I suppose.
They are good when there aren't a ton of Pyros on the other team. When there's too many Pyros, then it just ends up being Pyro vs Pyro, when only depends on who hits who first. . . No running away possible.
They're also good to uber when a whole room needs to be cleared.
For general gaming, I give them a 5 / 10. Just my opinion though!