Buy a pre assembled kit and you toss it in your car, pack ...whatever and will probably have to learn to use them when you need them the most. Not the optimal situation.
I'm with Hookpunch. I have a hard time personally identifying with the idea of buying a pre-assembled survival kit. I build up my kits mostly from what I have around. Raid my wife's sewing box for a whole package of needles. Raid my sewing box for #69 bonded nylon thread. Spool of stainless steel wire in the garage, next to the large role of duct tape. Etc...
To be honest, I don't really have "kits" - I have "gear". Most of it it gets used at least from time to time. I don't want cram it so tightly into an altoids tin that I'd almost rather freeze to death than contemplate digging into it and then re-packing it.
"What if you get stuck with just what's in your pockets?" Well, ferinstance, I have a bic in one pocket, a flashlight in the other, and I know they work because I used them whenever I need them.
Doug's kit falls into the category of "better than nothing" for people who might just as soon carry nothing. Lesser quality kits start to edge into the "false sense of security, worse than nothing" category.
Read some of the older kit reviews on the main ets site. It's an eye opener. Whistles that don't whistle and empty match cases are two examples that come to mind.