I had never heard of this Japanese store until a wonderful Facebook thread. The poet Joseph Massey asked what notebooks people write in, & being writers, we had our opinions. Several mentioned Muji, & one branch turned out to be just a couple of blocks away, on Cooper Square near the Carl Fischer music store (that is something else now). I went over this afternoon.
Didn't love the notebooks, mostly because I prefer lines, but I did buy a pocket one ($2.25). I sat on a bench & inaugurated it with a little poem. The best things I got were a key ring/notebook (99c) and a bookmark/pen holder ($4.75).
Update: I went back a couple of days later & bought more of everything plus pens. Lost the little key ring notebook, however.