The kits will gradually lose their punch, but shouldn't spoil. When exactly you'd notice a difference, I don't know. Certainly months rather than years.
I'd dismantle them: put the yeast in the fridge, hops in the freezer, crushed grains in a cool basement. Canned extract, probably in the basement as well.
The beer probably won't suck even if you wait a year to brew with these, but fresher is better. So I guess it depends on how good a deal (1/2 price? maybe...) and how often you'll brew. Also, I do think you would want to try others. I prefer AHS, Northern Brewer, etc. kits to True Brew.