Tom - I have seen ads for "all solid state" transponders, but they are pretty new. For the sort of pulse power needed at 1GHz, thermionic devices are probably preferable, and are actually pretty robust. I'm still using a transmitting valve here that has "1943" and the W^D symbol on it.
Dan - are there any SSR units on airfields, apart from the one at EGLL? I thought it was all piped from remote SSR units these days.
Anyway, SOP in our group is "lights - camera - action" when taking off. If I'm in the air, Mode C is on. Nobody ever complained, and some military folks in Scotland seemed very glad of the Mode C...