I was sent a Leuchtturm1917 notebook to try, a few months ago. I've tested it extensively (as in, I carry it around the office like Linus and his blanket, and write work-notes in it fairly constantly).
Apparently, it's gaining traction as a Moleskine-Killer, and I can see why. They are expensive notebooks, but they are very well made and they have some really interesting details.
My favourite parts:
-the pen loop (sold separately). Just right for my Uniball Micro (fine) pen, which bleeds through a tiny bit but not enough to notice. I don't keep the pen in there when I'm at my desk, but if I'm moving around, it comes with me.
-the hard back. I can write anywhere.
-the binding. It lies flat and doesn't turn the pages by itself.
-the feel of the paper - it's really smooth.
-the size. It's the "Master Slim," which is pitched as A4+, so that you can put sheets of A4 inside without them getting crumpled at the corners.
-the dots. I'm not sure if you can see them in this picture, but rather than a line or a grid, there are dots that I can use to make diagrams, draw charts, use as lines to keep my writing straight, or doodle on. The paper reads as "plain" but I end up with nice neat lines of writing.
My only complaint so far is that I've noticed a tiny tear in the (faux-leather) cover, just on the spine. Maybe some superglue is in order...I don't really want the tear to get bigger and I have quite a few pages left.