The Nikon 5T and 6T multi-element diopters (which Nikon calls "close-up attachment lenses") are 62 mm. The 5T has a focal length of 26.3 inches (667 mm), and the 6T is 13.1 inches (334 mm.). They can be used together, in which case the 6T should be attached to the lens first. The 5T can be considered as a +1.5 diopter and the 6T as a +3.0 diopter. The 3T and 4T are similar, but smaller diameter, for 52 mm. lenses.
There is no question in my mind that the optical quality of these Nikon lenses is superior to most, if not all, single element diopters. However, there are limitations. The multi-element design is unavoidably thicker than a single element diopter. If you are using the 5T or 6T with a wide-angle lens and a dome port, you may get vignetting in the corners and sides from the larger multi-element diopter. That won't be a problem with a reduced frame format digital camera, nor, of course, if you are using the CU lenses to achieve a greater magnification with a macro lens, but it can be a problem with a any full-frame digital or film SLR. The other limitation is that these lenses only cover 52 and 62 mm diameter lenses.
I used to use the 5T (and 6T) with the 20 mm and 28 mm prime lenses for wide-angle, and liked the results very much. Since I now mainly use the 77 mm 12-24 for wide-angle (or the 10.5, which of course takes no diopter at all), there is no alternative to using a single-element diopter, or at least none that I am aware of.
I still use the 4T for super-macro with the 105 mm micro lens.
Frogfish (Robert Delfs)