I am going to offer a somewhat different perspective. I don't use an ultralight pack. Mine weighs in a bit over three pounts--but it also doesn't come with a warning. Most of the ultralights will clearly admit that these packs are made of very lightweight materials, and will not stand up to abuse.

I'm not saying I abuse my pack, but if I want to put it down on some granite, I don't want to have to think about it.

I'll end this by asking a question: for those of you who use very light packs---how many miles/seasons do you get out of them? And do you feel that you have to "coddle" them in any way on the trip?