I think it's mustachian for me, to a degree.
I love, love, love my kindle - I have the kindle keyboard (old version!) and it has lasted for years. I used to travel every week for my job (how unmustachian is that, flying to work every week?!) and my kindle allowed me to bring a virtual stack of books that I could fit in my purse. I'm a voracious reader, so I usually read several books a week.
I carry my kindle with me basically all the time, and use it daily. It's fantastic when traveling, and it allowed me to get rid of 4 large floor-to-ceiling bookshelves (replacing a bunch of those books on the kindle wasn't particularly cost effective though. ahem).
I also re-read books often, so anything I buy will be read many times. I do buy a lot of books, but I also read a fair number of free books, and borrow some through Amazon Prime, and I use the "lend" feature for kindle books when it's available to borrow books that my mom and sister purchase.
I don't think they're for everyone - but if you read a lot of books, and/or travel a lot, it probably makes sense!