Here's a quick review of some notebooks I tried from Muji. Global note : 10/10 !!! Here are some scans, so that you can judge by yourself. These notebooks are very thin, which I like, and feature lined pages. They exist as well in "kraft" color if you prefer, but the dark blue cover suited me better.
Here are the three sizes in which I purchased them. A smaller size exists, but it was too small for me. (The biggest I bought was A4, the two others are A5 and B5 I think...)

Here's a picture of the back of the middle size one:

But the best part of them is the paper...completely FP friendly! An outstanding quality... Judge by yourself
So here are some random words. written on the smallest notebook with my VP 0.6 Binder Stub. The black ink is Pilot Black cartridge, and the blue is PR Midnight Blues.
Don't mind the sentences (some of them are in french as well...)

Finally, a quick handwritten review on the A4 size!

And the back of the page on which was written the handwritten review:

All right, I hope you enjoyed my little review. I'm sorry for the scans, but I don't own a decent DSLR to make decent pictures. If you you want more informations on these, feel free to ask!
Oh and yes...the best thing...The biggest size (A4) only costs 2.5 euros, and the smaller size are even cheaper
Have a nice day!
Shinichiro.
Edited to add: Sorry for the scan of the handwritten review...the image is not straight and the right side is missing sometimes...Also... the title is in french... but it just means... "Writing with a dip pen : First Volume".