I'm cheating here, because I don't have a MUSA Liner. I have the one you used to sell -- the orange one from Georgia. But I don't suppose the two differ much in performance.
I can't decide whether I love or hate my vapor barrier liner. It does exactly what it's meant to do, and on a colder-than-expected night that counts for a lot. But it's slippery, and that can be annoying. When using it I have to take extra care to pick a level campsite, or else I'll be fighting gravity all night. And sometimes I like to lie on my back with my knees bent, but that doesn't work with the liner because my feet keep sliding down.
I take my liner on every trip, and I use it when I need the extra warmth. But when the weather is mild enough, I'm glad to sleep without it.