As a pyro, it is YOUR SWORN DUTY to spy check a lot, especially near engineer's buildings.
The compression blast is great for blocking projectiles, trust me, but have a dispenser nearby.
The backburner loses as it gains. You lose compression blast and yet you gain the power to burn people from behind better.
Don't forget, compression blast DOES EXTINGUISH FIRE ON OTHER PLAYERS.
As a medic, always be on the lookout for a "MEDIC!" bubble that is on fire, this means that the person is on fire and needs help.