I have only used the 21" version and can say it cuts very well. The saw is big enough to get a good cutting stroke and still stores with minimal bulk and is not terribly heavy. You can really get a nice stack of lumber with this saw. I would like to try the 15" model. But, I have not had an opportunity yet. I would expect it to perform comparably. If I were to own one I would likely look at the 15" version just in an effort to cut a few ounces.
The debate over whether a fire is necessary (or even should be considered) will likely smolder for many more years to come. While I agree that a fire is not needed, it can certainly be a big morale boost and be very useful. If you want a "real" saw to assist in making a "real" campfire in the backcountry, the Sven saw is worth a look. If you are a real gram counter, there are lighter options available. But, the weight savings will no doubt be a compromise to what it can cut, and how efficiently it can do so. If you are a true ultra light backpacker you probably quit reading after the first paragraph - If you didn't you probably should have.
Final verdict: It's a great saw. But, it falls in the nice to have category for me. It's something I would consider taking on a group trip where duplicative items (weight) can be split up among multiple packs or, on short trips where I really wanted the luxury of a real-deal campfire.
For more information visit their website at http://www.svensaw.com.