TL;DR: Cheap, but works.
I don't actually have any "cons" because I was realistic about my expectations going in. If you don't ask too much, you can be pleasantly surprised.
Not wanting to break the bank with a few hundred dollar's worth of sleping bags, the wife and I used these in conjunction with some other gear to make a rather lightweight, very effective camp bed. We're both small adults, so we had plenty of room inside once we zipped them together into one mega-bag, put that on a sleeping pad, unzipped an older summer bag and used that as a big blanket over that whole setup. We were toasty warm overnight into a 30F Oregon morning at about 6000 feet. Our bedding was covered in a layer of ice, but we were comfy... until we had to get out.
I'd recommend them, but mostly as a convenient-to-use component of a larger setup. As an only-take-this-bag kind of thing... yeah. Warm nights only, but it'd be great for that since they are so light to carry in their stuff sacks.
As others said, indoors would, of course, be a fine use for them as well.