I picked up a manmelter last night and I love it. I've yet to hit someone with a critical hit but being able to switch to it for extinguishing removes a lot of ammo management from the backburner. Although in an ironic turn of events despite jumping onto a Valve server to test it out there were no phlogs the entire time I was playing.

I agree, teleporters are the most useful buildings in the game.
A fun teleporter story.
Back in the autumn I set up a second steam account. My wife had expressed an interest in playing TF2 and I figured I should get her some gear - although things got busy and nothing ever happened with this. Anyway, when I set it up I messed around a bit to get some drops and decided to play an engie since it's not one of my usual classes.
My second match as an engineer I got nearly 100 teleports without dying. This was on the second round of Dustbowl, attacking, with a nest at the end of that side tunnel. I had less than half an hour of play time and managed a score of 50 points. On my normal account my top score is 42, obtained playing pyro - a class I've sunk more than a hundred hours into.

As with extinguishing teammates, a wellplaced teleporter makes you a ton of friends. Turning it around so people show up facing the action instead of with their back to everyone also got me some compliments. They really need to adjust it so an engie can easily see which direction their teleporter is facing after it is placed, I think it would help a lot with this problem.