I ended up buying both 8-12x42 and 10-15x42 new, with the intention of testing them out and returning them. Overall I felt like the quality of glass was excellent. I would give the slightest edge to my Zeiss conquests for having slightly better contrast to my eyes. The fit and function of the duovids was excellent—very smooth focus, sturdy eyecups, etc.
The 8-12x42 did not really add anything optically to what I could see with my conquest 10x42. Plus, the FOV was about the same at 8x on the duovid as it was in my conquests. The 8s were definitely easier to hand hold which was tempting, but I just didn’t feel like 12x was enough of a step above 10x. Previously I had looked through razor 12s compared to my Zeiss and preferred my Zeiss, and did not think the extra 2 power did a whole lot for me.
The 10-15s were pretty sweet. On 10x, the FOV is not very good unfortunately. It’s not a dealbreaker, but I do enjoy the expansive FOV of my conquests. The image sharpness was pretty much identical, as was the low light performance. Both did well past legal shooting light. On 15x, the duovids are awesome. Very bright, crisp, clear image. I was worried that the smaller objective and older coatings would disappoint but they did not. I would expect some of the newer 15s to perform slightly better, but to me the utility of having both bins in the same package outweighs the slight optical compromise. I wish Leica would update these things but I doubt there is enough market for them. In the meantime I am looking for a used pair to try in Wyoming this coming fall.