I keep a few small and a few medium sized ones in my gear repair kit for doing quick repairs on the fly to gear or equipment.

I find that their single use nature makes them less practical in a survival setting (IE: You use them to help lash a shelter together, then move on due to weather or whatever...and you need a whole set of new ones when you get to your next location). I much prefer utility cord, rope or paracord for such things...much more practical.

I've experimented with the zip ties which you can release but they don't hold strongly enough to do anything serious with so I use those as cable ties on my computer and stereo at home.

+1 on the Thomas & Betts (or similarily designed) ties. They may seem like overkill but they'll hold under way more load than the cheapies before stripping and the mounting head as a 101 uses.