During the time with my Nikon D300 I always used regular (thread) filters (circular polarizers, and ND filters). Since the release of the Fujifilm X-T1 I wondered if a Lee filter system might be better / more flexible (not cheaper!!!!).
The Seven5 series is cheaper since it uses smaller filters (75mm versus 100mm), and since my Fujifilm X-T1 uses relatively small lenses this could be a winner (the kit lens has a 58mm filter thread). Until I found out that the new ultra wide angle Fujinon XF 10-24mm F/4 R OIS has a 72mm filter thread. And as you might guess, I'm really interested in that lens.
Fortunately, Lee has a 75-to-72mm adapter, so technically the Seven5 system can be used with that lens.
Adaptor ring thread sizes:
The holder attaches to the lens via a screw-in adaptor ring. The adaptor ring is available in the following thread sizes: 37, 37.5, 39, 40, 40.5, 43, 46, 49, 52, 55, 58, 60, 62, 67 and 72mm.
But 72mm versus 75mm doesn't leave much room on the vignetting side of it. Chances are that you get serious vignetting on the ultra wide end of the focal range (10-14mm), because of the filter holder attached to the lens.
Just to make sure, I dropped Lee an e-mail, and this is what I got in return:
So, there yo got it; Accept additional vignetting on the ultra wide side, or invest in the more expensive 100mm filter system. But before I even invest in a filter system I need to see some independent reviews of that new lens. I might even get the Fujinon XF 14mm f/2.8 R. That lens is available at the moment and is highly recommended by several sites  / reviewers / users.
Choices, choices, choices
UPDATE: After much deliberation I bought the Lee 100mm kit with two adapter rings. One for the Fuji 18-55mm (58mm filter thread) and one for the Samyang / Rokinon 12mm f/2 (67mm filter thread). I also added the Big Stopper (10 stops ND) and the Little Stopper (6 stops ND) to my cart.
So in the event I decide to switch camera brand/systems with different lenses (filter threads) I can still use this filter system. I only have to get new/other lens adapters.