I use it on a DZ, i.e., at f/12.8 with a Baader T-2 Zeiss prism diagonal.
It is (as others mentioned) invisible, and I like that in the case of the DZ, the Q1.6x (TKA36595) is a 2" accessory that you simply put before the diagonal (i.e., not having to screw it on/off).
The view from DZ+Q1.6x with the 12.5mm Docter (at x102) is absolutely stunning. Extremely sharp stars on a super flat field, it's gorgeous.
I *think* DZ+Q1.6x with 6mm Abbe (at x213) is ever so slightly sharper than DZ with 4mm TOE (at x200). I tried this comparison few times, but seeing must be really good to make a conclusive statement. I got a hint of this few times, it is extremely subtle, and without the ability to go back and forth it would be extremely difficult to notice any difference (the DZ and TOE are very sharp optics).
I also use it (for fun) with a 3.4mm Vixen HR at x376. Izar the other night was an orange and a blue ball separated by a black sky. When the seeing allows such observations (rare here in New England), it becomes much easier to see color differences between stars if the Airy disc is large.