I also like using it with the helicoid—if you look through the pictures on the Flickr ZM C-Sonnar page you’ll see many I have made doing so, most between f1.5 and f2 (where the sharpness is excellent on my copy). While I rarely make images that aim for “everything sharp”, I have also used the camera for a fair number of more infinity-range landscapes. It can be quite good on some of these, but also merely adequate to mediocre. I find the outcome has more to do with my care in focus and composition than the lens, as they are nearly never images that would have been improved by merely being sharper.
I took a trip out to the Oregon coast in January to visit family, and many of my images are from this lens—almost all are some sort of landscape/nature. Some are on the Flickr page. Frankly, I don’t think this lens limits my ability to do the kind of landscape photography I like, and the rendering actually facilitates and makes it possible. The textures in particular are really lovely.
I use the Fotodiox Pro close focus/helicoid adapter for a long while—it’s very smooth and works well with a tab. I recently picked up a used Voigtlander close focus, which is a bit tight (just needs to be used more, I think) but smooth and slightly more compact. Definitely heavier. Both work just fine.
I still highly recommend this lens. It’s staying in my collection for sure. I rarely use it beyond f4, and mostly enjoy it between f1.5 and f2.
Cheers,
Daniel