I reported on the Zeiss 3X loupe, Zeiss part number 522 012 back on January 21 with respect to the Pentax 645D. It’s somewhat helpful on the Pentax 645D under the right conditions, but not so great in general due to the slow (dark) lenses (f/4 or f/4.5) that make the 645D focusing screen quite grainy.
But on the Leica M9 , the 3X magnification is extremely helpful! There is no ground-glass to go grainy and dark; instead you get a fully magnified image at 3X which makes precision focus unambiguous and obvious.
It’s not so easy to focus handheld with the 3X loupe, but it can be done, and on a tripod I now consider the 3X indispensable for critical work, which of course assumes that the M9 and lens focusing mechanics are accurate to begin with— not a given even with brand-new gear.
I have Zeiss part number 522 012. I’ve been bugging both Zeiss and B&H Photo repeatedly to get this in stock at B&H, but I guess they don’t want to sell it very much, because it’s still not there. Here’s a page with Zeiss 3X loupes at B&H Photo.