I have WeatherTech floor liners in my other vehicles, but opted for the 3D MAXpider Kagu liners for the CX-9. The MAXpider product is made by a Taiwanese company which (I believe) also makes the Mazda Canada all-weather floor liners that appear to use the same materials and process.
So far, I am pleased with the MAXpider liners. I think that the texture of the liners blends in well with the interior carpet, and makes the mats more subdued and less noticable, which I personally prefer in a more upscale vehicle. The tiny "hook" material on the bottom of the mats works as a mechanical "glue" to ensure the liners adhere to the floor and sides, so that they don't slide and gap as WeatherTechs do. Everything fit very well with the MAXpider liners, with two points of irritation:
1 -- The second row liners do not extend forward quite as far as the factory carpet mats. If a taller person is seated in a second row seat and extends the tips of his/her toes under the front row seat, the person's feet will overhang the front of the liners. This isn't necessarily a big problem except in the winter, when snow (and salt) could melt and drip onto the floor under the front seats, escaping the containment area of the liner. Comparing the shape and size of competing liners online, it appears that most or all of the competitors (including the WeatherTech and Mazda Canada liners) have the same issue. In fairness, this might be a difficult problem to solve, as extending the liners too far forward may block the floor HVAC outlets under the front seats, but I still think that the mats could be extended a bit further to be comparable with the factory carpet mats.
2 -- The MAXpider cargo liner isn't quite a perfect fit, and therefore bulges slightly in the center due to pressure from the friction fit on the sides. The problem areas appear to be around the rear side bin "ears" and along the plastic trim below the third row cupholders, but I suspect that storing some weight in the center of the liner for a while may cause it to settle in. If it becomes too bothersome, the sides are easy to trim, and there is ample material outside of the perimeter containment ridge to loosen up the fit without affecting the liner's ability to contain liquid. I'm not a huge fan of the molded pattern of the cargo liner, and would probably prefer it to resemble the subdued pattern on the passenger floor mats, but perhaps the pattern was done to help prevent movement of cargo.
With that said, I've been happy with WeatherTechs in my other vehicles, but they *do* tend to curl on the sides over time, they don't anchor well to the underlying carpet, they stain in a way that no amount of scrubbing will clean (unclear if this will happen with the MAXspiders), and they have a slightly more utilitarian appearance. I believe that the WeatherTechs are made in the USA, which is a huge plus in my book but may be of varying concern to others.
I don't think that I would like the Diamond mats because I don't want the mats to stand out as a feature of the car, at least in the CX-9. I suspect that the material and stitching wouldn't be as easy to clean or maintain as the synthetic plastics and rubbers used with other mats and liners. However, if you want to show off your floor mats, the Diamonds or TuxMats mats may be more to your liking.