I have and use a pocket chainsaw like you're talking about. In my experience, it cuts really well on dry logs that are stiff enough to resist the pull of the chain, usually anything wrist sized or larger that is either close to a trunk, close to the ground, or right next to your foot. It can bind up pretty bad in green wood or if you get wonky in your cut. Also, it's tough to cut wrist sized stuff that is unsupported (eg dead fall) or anything smaller, b/c the wood just bounces with the pull and the saw doesn't really bite like you want it to. If you can hold the piece down by wedging it or stepping on it that helps. I've found that I can usually cut the same amount of wood just as easily with a folding saw at about the same weight, so for me it just depends on the day & conditions as to whether I take it or not.
A couple of things I really like about it are the ability to cut really large logs, and the ability to tie rope/cordage to the ends and cut at a distance with my body weight if I need to.
I wouldn't hesitate to put one in a car kit, as you could cut some really large logs (say downed trees across the road) if you had to.
BTW the wire saws can be very effective with the right technique. I personally thought they were a gimmick until I saw some vid's from folks on the WSS forum showing how to use them.