I owned the doubler and took it back. I also own the 10x42's and the 15X56 Swaro's.
I found a route I like better. It is still not as good as a spotter (I prefer the Nikon 50 MM ED if you are backpacking).
However, if you want to lose and extra 8-12 oz. try one of these:
http://coueswhitetail.com/bookstore/tripods_adaptor.htm
The eye relief is not as good as the Swarovski doubler, however, there is a wider field of view on the Jim White binocular magnifier.
If you are not satisfied with it, Jim will refund your cash. It is $299 and I feel it is a better product. It slips on rather than you have to unscrew everthing with the Swarovski doubler.
I think it actually works better on the 10x42 than the 15x56 (Due to larger FOV)
It works reasonable well if you are backpacking and don't want the extra weight. Not great for serious trophy evaluation, but for good detail with 1-2 miles. Obviously, use with a tripod. I take it with me in the field with my 10x42's and leave the heave 15x56 with in the truck or in a locked quad box.
Just for kicks, I slipped it on over my 50 mm ED Nikon spotter and was shocked that it did as well as it did. I have the 40x zoom and it turned it into a 80x+. Granted low light was not real good, but the magnification and clarity were much better than i thought they would be. With this compo I can definitely evaluate a trophy out 3-5 miles just fine (except real low light).
What is great, with this combo, I have great clarity with the 50 mm ED at 40x. If i slip on the 2.5 magnifier, it goes way out there! All this in just 27-28 oz. Tell me where can you find an 80x zoom with that kind of weight?