Key point - the lens only renders well across most of a full frame when focus on the host lens is set at or near infinity. Closer in, more and more of the edges get bent and not so sharp. This means you cannot get a sharp, full frame image at the theoretical max of 0.86:1 on a 70-200 set at 200mm. About 0.6:1 is the practical max for that (at 200mm). It's best to change reproduction ratio with the zoom control on the lens, not focus control - although that can be used more down around 135mm or shorter (with less softening and field curvature happening around the edges). AF even works quite well there, within the 30-50cm working distance range.
Having said that, when used within it's few sweet spots (or zones), the lens renders very, very nicely. It does not hide the character of the host lens in any way (including bokeh) and only softens the image toward the edges. It softens badly outside the sweet spots, even smears in the worst cases. The center is as sharp as the host lens at all times and there are no added aberrations. I would suggest this NISI close up lens is best considered for APS-C if you want sharp across the frame. But for more "arty" shots, it's perfectly adequate, I'd say great, for full frame too. Just work with it.
I love the long working distance and perversely, I'm glad I couldn't really use the theoretical max mag. It would be too much when trying to track bugs via the viewfinder. 0.5x and less is better for a while. Easier to practice and dial-in the muscle memory for more mag (maybe) once skills permit. The handling and feel is taking much adjustment though. It feels halfway between working handheld with a big prime (where you tend to have a tucked, hold it close upper-body position) and handheld with a manual focus macro like the MP-E65 (focus-by-moving, and a much looser, random upper body positioning from doing that). The weight of the Nisi on the front is very noticeable in this scenario too. I did strain my elbow a little (just felt slightly sore later that day) but mostly from over-reaching, one-handed. I think it will be fine if I stop doing that.
Less than three years ago I'd have tested the lens from a technical performance perspective (test shots and pixel peeping) and then told you it was a pile of poo, run away, don't buy it. If you want a sharp-across-full-frame, useable at all magifications lens, then that's what I still say to you. But if you want a cheap, arty LWD macro option with some eminently useable sweet spots, a different "feel" to shooting with it, and it's related set of different suitable targets, then it's well worth a look - providing you already have a suitable lens to host it. Just don't expect perfection.
In summary I'm really pleased with the lens and can live with the limitations. This is mainly because I had a narrow set of subjective requirements for it, and it meets all those admirably. Otherwise I may not have been so positive. Reasonable value for the money IMO (£110). I'm sure there are better ones, some cheaper, some more expensive, but this one complements my other gear nicely and should do me for a few years. A keeper.
187mm, 1/200th, f/9, ISO 320
200mm, 1/250th, f/7.1, ISO500
118mm, 1/100th, f/10, ISO500
200mm, 1/160th, f/10, ISO500