I finally got one of these to try out, and I have to say that on tenor, it made a big difference. (So much so that I'm using it to replace my Optimum on tenor.) It's not THAT difficult to use, and the sound is totally worth the effort.
The main problem with a perpendicular (direct pressure) ligature is getting the amount of pressure correct. You want enough to seal the reed, but not so much that the plate digs into the reed, which in my mind is worse than binding fibers because it actually damages the reed. This is a real problem with Optimum and Winslow ligs. (When you take your reed off, if you can see impressions on the reed where the lig contacts it, you have this problem.)
The FF has the opposite problem - not enough pressure. If your reed table isn't perfect (and I do mean perfect.) the reed isn't going to seal with it. Fortunately, I'm using a Morgan, so it isn't a problem, but I'd say with a lot of mouthpieces (especially non-perfected stock pieces like a Link or S-80) it's probably going to be a large problem. I'm not sure how well it will work on a concave table piece either. Unlike a fabric lig or a Winslow, this ligature will not compensate for your mouthpiece or reed.
However, the sound with this is much clearer and more "natural" sounding with the FF, and staccato is much easier to perform without the fear of chirps or "back tonguing". I was really surprised at the difference and very impressed with the sound. The response was improved tremendously, and my Keilwerth finally felt as free-blowing as it should. Altissimo was greatly improved also...no more struggling for high notes.
The only other criticism I have is that the lig isn't supposed to stay on the mouthpiece when you aren't using it and the "smart cap" is actually rather stupid. Fortunatley for me, there is a dedicated spot in my case for the mouthpiece, which allows me to keep it on. It also looks a little odd in use, and it reminds me of orthodonic headgear.
If the sound is going to be improved this much, I can live with the problems. It still needs another screw at the bottom, but otherwise I found that it works very well and really opens up my sound! If you're looking for a new ligature, try one of these and see if it's good for you...they are a little expensive, and they aren't for everyone, but I really like mine and I'm using it now as my primary classical lig on tenor.