I say it's an excellent intro to someone with maybe some academic programming experience to get into web app development. it gives you the kick starter development plan you need to make a Twitter, db, grub hub, air bnb type startup alone or with a small team.
and... he keeps it up to date! with impeccable detail!
In fact I'd go so far as to say that trying to learn Ruby first is doing yourself a disservice. Just dive right in.
I see so many people just skip Ruby fundamentals and go straight into Rails, or worse... they think that Rails is... Ruby.