I, too, shoot Leica for landscape, and can confirm that it is both wonderful and a total pain in the ass.
On the wonderful side, it's a real camera and feels that way in the hand (not a plastic p-o-s), is far more compact, and produces stunning images. I feel that there is a noticeable difference in quality between the M9 and the 5D2. It's a *qualitative* difference - the images have their own visual signature to them, which I prefer. They are much sharper at wide apertures and at frame-edges. This is especially true with wide angle lenses. The Leica lenses are awesome, the Canon lenses suck at the edges and corners. The AA-less CCD chip produces lovely images - especially at low ISOs.
Put it this way: I would use the Leica for serious landscape photography, but not the Canons (if I'm doing dslr, then it's MF-or-bust for serious landscape). The overall micro-detail and quality of the image on the Leica is finer, with none of that annoying, plasticky feel the Canons often deliver (probably due to the AA filter and the internal noise processing). I can't comment on the D3x personally, but if you're going to carry that kind of weight, might as well go MF.
On the pain-in-the-ass side, it's irritating to have to change lenses all the time, and the framelines are awfully inaccurate. This means shooting, chimping, move the camera slightly, reshooting, re-chimping, etc. That's just the price of using a RF.
Overall, I love the Leica for this kind of work, but it's horses-for-courses.