I'm glad we've gotten on this subject, because as a new player I must admit ignorance here.
Why would I want to use anything but the Uber medic gun? I understand that the other guns charge faster, but their effects seem so weak in comparison that (to my unexperienced eyes) they seem crappy at best.
I can remember numerous times being followed by a medic with full ubercharge to the frontlines, they pop kritzkreig... and I can't push past the sentry defense because the healing isn't nearly significant enough and I get no bonus damage against sentries anyways. Or I get a kritz popped on me and the opponent pops an uber in response. I guess that means we run now? Thanks for the charge though I guess?
Also, I like ubering heavies over soldiers at times solely due to ammo. It happened to me as a soldier the other day on the last point in a payload map, I get ubered as a soldier with 2 rockets left. OK I guess that means I pop out my shotgun for the last few seconds. Not the end of the world. Oh wait I got another uber right after the first ended with no chance to reload? Well this is going to be awkward.
Once again, total scrub viewpoint, just looking to learn here.