They turned the Ambassador into garbage with this last patch, and made the Dead Ringer easily the most fun (and probably overpowered) item in the game. When I get on a roll with the Dead Ringer, I'm completely unkillable. The only thing that worries me at all are Pyros and even encountering Pyros is no longer instant death. I play a lot of Spy and this made the class 10x more fun. I haven't used the cheap invisible taunt yet but even so it's just beyond amazing how hard it is to kill me.
I probably will end up using the cheap invisible taunt, too, to get rid of sentries with camping Engineers. I'd gotten used to using my Ambassador to headshot an engineer twice quickly which could kill them faster than their dispenser could heal them, even through objects. With the headshot nerf and the inability to now fire through objects they've given Engineers a little too large an advantage over what should be their hard counter.