They are light (at an A5 form factor), have a very good paper quality, and covers with textures that fit comfortably in the hand; they also fit comfortably in my pocket.
(there's also some with hardcover, but I find them too heavy)
Moleskin also make grid-paper and non-ruled notebooks.
Personally I have seen the light and find that Moleskin paper is not the best. I switched to using Japanese paper. I'm left handed and Japan needs fast drying pens and paper.
Also the Whitelines app with this notebook is really a great idea. You basically take a picture and the grey background is removed and you have just your writing or drawing.
(there's also some with hardcover, but I find them too heavy)