SPUF has pretty much agreed: we hate the gunslinger. I am not going into a rant about whether it is overpowered or not – check any page of SPUF for that. Regardless of whether or not it is OP, it is still no fun to play against. But it can be countered by most classes without too much effort. Of course, you can always call a friend over to help you take it out, but sometimes you just have to do it yourself.
The mini-sentry has 100 health and 48 DPS, which is formidable considering it is such a small target. Many times, your crosshair is bigger than the sentry you are looking at. It builds faster than it takes a dominated machina-toting gibus-wearing sniper to ragequit and fires quicker than a hatless spy frantically firing his revolver at an overhealed heavy.
This guide also covers tackling an engineer/mini-sentry combo, where the engineer is hanging around near his mini-sentry so he can constantly feed spam to the other team.