I have and have used the pocket chainsaw and the saber saw (the new version with cutting teeth in both directions). The new Saber saw I think actually may cut faster. They will both kink and/or bind on you under less than ideal conditions. My preferred method for both is to stand atop the log, snake the chain under where I want to cut, and work the chain with my shoulders while applying upward tension. I have no problem going through a 1 foot diameter green douglas fir trunk in one minute per cut-through in this manner. I could probably do a cord of green fir wood like this in less than an hour, depending on how well staged it was.

BTW, you can make the Pocket chainsaw into a nice bow saw with a pair of limbs cut to length, trimmed and tied in a scissors shape, and brace locked.