I am a push-to-talk guy
i use the mouse wheel button for TS(mumble) and the left-alt key for ingamevoice chat.
i mostly do this for the obvious reasons, less chatter on the channels, and only what i want gets transmitted. not ringing of telephone,passing trucks on nearby street or the screaming (my and his) when my cat startles me while i m playing left 4 dead.
also it is easier to configure, no problems with having loud music running in the back or other soundloop problems.
of course the response time suffers a little, always reaching for the button to do say something obviously takes some time but i prefer it this way.