The book of shadows is considered a must-have for many pagans. I've certainly given it a try a number of times over the years, but despite my best efforts, every blank book I carry turns into a mishmash of to-do lists, scenes for writing projects, metaphysical notes and everything else.
The closest thing I actually have to a book of shadows is probably my evernote account. That's where everything interesting that I find online ends up: almost all my research, much of my personal writing, and notes I make while on my phone or otherwise away from my computer.
The important thing is what's inside the book, isn't it? I've talked about this "electronic book of shadows" before and had people tell me that it's not a real substitute because I'm not writing it by hand, or because I can't take it into ritual (which I can, actually - that's what laptops and phones are for). It may not be for everyone, but for me, it's a perfect fit to my needs and ritual style.
It also, like my many attempts at paper journals, is not exclusively for magical work. I consider this a good thing. My life doesn't have strict boundaries between "magical" and "nonmagical" so I don't expect my notes to have one either. This cross-contamination is actually useful - it means I'm often surprised by the results of a quick keyword search for a specific fact, and can lead to inspirations that I would not otherwise reach.