Been out with this pack three times now, logged about 20 miles. I have a fit 20lb. Boston Terrier, and the XS size fits him perfectly. No chafing, no problems at all. He's been through streams, in lakes, up and down boulders, mud, brush, you name it, that pack can take it. It only took him about five minutes to get used to it, too.
The handle comes is great for lifting him over the few obstacles he can't deal with.
I usually put his leash, some dog food, and a rain fleece in it. Sometimes water if there is no water source on the trail. A couple of times I have had to put a small rock in it to level it out. Never more than 2-2.5 lbs at the most, tho.
I have also put it over a fleece that he needs in cold weather, and it works just as well.