Okay, so if you see an enemy medic, no matter what else you see you kill the medic first. There is NO other option. Medic FIRST!

If you see a medic healing someone, kill the medic first. always KILL THE #$*&ING MEDIC!!!

If there is a medic on your team, defend the medic. The medic, when he gets UberCharge to 100 can make whomever he is healing invulnerable for about 10-15 seconds which can decimate the enemy.

Heavy + Medic with UberCharge = capped point if played well.

http://www.penny-arcade.com/comic/2007/09/21 says it pretty well too...