After having seen the quickfix used pretty extensively in pugs, it's certainly viable. I don't like playing against it, however. The mobility makes medics much more difficult to kill, and the speed of building the uber makes a medic pick mean less. And the heal rate is, obviously, much higher. "I hit that demo for 200!...oh the medics on qfix gl killing him." Now, that was just a pug, I think coordination afforded by a team would change how the team approaches these sorts of things. Between that and the equalizer being nerfed, I had a hell of a time as roamer just pugging.
It's cool that valve buffed the quickfix, and the buff is certainly viable and pretty balanced. But imo it does have a negative effect on the quality of the game.
Call me conservative, but I think some elements of the game are almost untouchable. The heal rate and survivability of the medic class are good examples of two.