Just received a Mafell KSS40 today and put it to work on the deck I am currently working on. I am very impressed with it's power, smoothness and weight. My decking material is S4S+Plus Cumaru, very dense and hard. This saw cuts through it like butter. I previously tried using my Festool HKC55 and it cut much more slowly and was leaving a rougher edge. The HKC also weighs considerably more than the KSS. I then switched to a stand mounted Dewalt Miter Saw which worked ok, but actually cuts the Cumaru slower than the KSS. This also required carrying the heavy decking boards to the saw for cutting. With the KSS, I am able to place by boards, mark them for the required cuts, and then cut them without hauling them back up to the miter saw. And if I need to trim one, it's a piece of cake with the saw mated to it's mitering rail. The miters are spot on, matching the 45's I've been drawing with my speed square, and mesh together nicely on the deck surface.
Hefty price tag but I wish I had taken the plunge earlier. I might have already completed the deck by now if I would have had this tool from the beginning of the job.
Looks like I'm going to be selling an HKC in the near future now. That, it's three sizes of rails and the bag that fits the 670 rail. I seriously doubt it will see much use in the future as I have little need for it's additional depth of cut, which is where it has a leg up over the KSS40.