Apparently a single-use device, once your hot water tank fails, replacing the Water And (optionally) Gas Shutoff valve is the least of your problems. It looks like it would be easy to make it resettable (water-activated life vests have reset/recharge kits), but I'm sure their lawyers threw a fit, someone rebuilding it wrong would cause Tasco to lose a lawsuit.
Of course, if your over-temp/pressure valve leaks, then it could get annoying. Probably best to install it with unions in case of nuisance trips? Still at $128 list that might get expensive, but Twowaxhack says there's a 'core charge', so that might help.
http://paragoninspects.com/articles/pdfs/plumbing/water-heaters/plumbing/leak-control/wags-valve.pdf
Kinda too bad they don't make one with an electrical shutoff, but then adding a Big Honking Breaker probably adds more complexity, and having an electrical element get stuck on is probably less bad than leaving the gas heater running. I'd settle for a low-voltage contact to tie into an alarm system, (or Raspberry Pi), but I suppose a dedicated water sensor is easier for Tasco, then they don't have to deal with UL at all.