Seek Outside has been using primarily XPAC in our pack bags for the last 5 years. We have used many colors and variants from 210D, Coyote, Olive , Camo versions and really really burly versions. It's good to see others offering it. Maybe Seek Outside is leading from behind in helping to improve the backcountry experience.
What we at Seek Outside have noticed is:
1. Noisier below freezing .. sort of like opening a bag of trail mix. Not much of an issue, unless you like to get really close to animals and then open your pack and dig around. Below freezing almost everything is noisier. Snow is crunchy .. Leaves might be crunchy ..
2. Slightly different type of noise above freezing than 500 D Cordura. The pitch is a little higher freq. How does this affect archery hunting elk ? Honestly, I have hunted with people using soft faced packs meant to be quiet. and I have been the quieter person .. and I have been wearing our loudest fabric. How is this ? Well, compress your pack down nicely, and tend the straps, and use your woodsmanship ability to walk softly .. also don't have the wind at your back or cast long shadows etc.
3. Blood and Cleaning -- it's a million times better with our XPAC packs. You can wipe it out really with a rag, but I usually put it in the wash machine without detergent super quick cycle and hang to dry. I've had packs that have had 5 elk packed out and my dogs show no interest after a wash and hang. Try that with Cordura. One of our packs even had gas in it from some forest cleanup chains saw use and it still cleaned up well.
4. Water resistance - The right bag designs will not need a rain cover. Yes you can seam seal them to make them more waterproof but .. for walking upright it is not really needed. In fact, I flipped a kayak in the Brooks range, and my kayak took a few hundred yard ride upside down .. and the contents of my pack had only a little water about the same as a dry bag attached to my Kayak. Some people have used our Seek Outside packs for keeping water out of an open ended tarp, and I even know people that have hauled fish in our pack bags.
Important to note: 500D Cordura packs can gain close to 1 lb in water weight and take a long time to dry (days). XPAC can gain a couple ounces in the fabric but it never soaks through and thus it dries much faster and can dry well in a small amount of sunshine.
5. Durability - 500D Cordura is really good but similar XPAC fabrics are pretty close. Lighter fabrics in XPAC like 210 D will not have the same rock abrasion and can get pinch cuts like any light fabric (210 Cordura included). The Lighter XPAC fabrics do fine walking through oak brush etc .. but don't use them on a technical rescue at 14k.