One of the Muji double ring notebooks that caught my eye was this one, a 5x8 notebook with a vinyl folder on the outside for pens and papers as well as a gray cotton elastic band to keep the notebook closed.
Inside is pale ivory paper with gray/violet dots instead of lines, very Rhodia dotPad. The paper isn't Rhodia, however, as there is quite a bit of bleed through with fountain pens ink,
I tried out a number of gel ink pens, some Sharpies and Staples OXO pens, and a few different fountain pens.
As you can see, almost all had a heavy shadow on the back side of the paper if not total bleed through. This is paper meant for pencils and ballpoint ink, or someone who doesn't mind writing on one side only.
I like this notebook, however, and plan to take it along with me for work notes and jottings. The biggest problem next to the paper is that the notebook won't lay flat if you have pens inside the vinyl folder.
So basically it's a great idea that looks good, but execution needs a bit more thought.