FWIW, I've had a pair of synthetic insulated pants for years, and while they seemed like a great idea when I ordered them, in practice I always find that the weight and bulk keeps me from carrying them. I'm more likely to throw in a pair of silk longjohns plus very lightweight rain chaps; the combination is lower weight and lower bulk and more flexible.

I'm not saying that I never use them, just that the right combination to do so isn't common based on my personal hiking style, trips that I take, etc. So maybe they will turn out to be a wonderful purchase for you!
Brian Lewis