ePhotozine published the full Sigma FP review:

The Sigma fp is a unique camera in a world of similar looking cameras, and whilst it’s going to seem “odd” and different to most people, this will be seen as a benefit to others. The modular approach is a bold move and gives you the choice of what you want on a camera. Don’t use flash or need a flash hot-shoe, then don’t take it. Without the added grip, the camera can feel too small, but this is something that will be down to personal preference, as well as depending on needs and what lens is used with the camera

Who is it not for? Die-hard Foveon sensor fans who believe in the Foveon sensor technology aren’t going to find what they’re looking for here. It’s also not for those who want a “standard” camera. Focus isn’t the quickest, so this could cause some missed shots, and in a shoot where you might want to quickly take a number of shots, the camera is noticeably slower than even an entry-level mirrorless camera, unless you’re using the high-speed continuous shooting mode.

Is it any good? The shooting experience, for stills photography, lends itself to slower shooting, such as landscapes, and architecture, rather than rapid photography, and the slow focus certainly limits you when shooting, depending on your subject. People who are used to other modern camera systems will be disappointed, perhaps, if they’re expecting a camera with rapid focus and shot-to-shot times. Stills photographers used to shooting with an OVF or EVF, may also be disappointed by the lack of EVF available for the Sigma fp. You’ll also find that the L-Mount lenses available, so far, are mostly quite large, and therefore feel unbalanced in comparison to the small camera body, unless you’re using the 45mm lens. The optional large grip certainly helps here.

This is where the CINE side of things come in. It has a modular camera body, which will be an exciting option for videographers, who seem intent on putting a cage on a camera the instant they get it. The Sigma fp is perfectly suited for this, and as a mirrorless camera, you can also attach an adapter, and use Canon or other lenses on the system. The fact that it is the smallest full-frame camera available will be enough for many, making it a great option.

Amadeusz Andrzejewski told me this:

I completed a video mode test of the Sigma FP and found some issues with the quality of the DNG video. The scaling of the raw data from full sensor width to 4K / HD seems to be implemented imperfectly. Some other testers I spoke with confirmed that. I’m attaching a screenshot, and the video is here:
https://www.youtube.com/watch?v=AlFncWktEl8
(it’s in Polish but with English subtitles)