I don't think learning Rails right now is a bad thing. I've been doing some side projects in it for ~2 years and definitely don't regret it. As some other folks in here have mentioned, there is always going to be something newer and better out there. If Rails is your first or even one of your first web stack experiences, it is a great way to learn the fundamentals and it makes learning the next new thing that much easier. In short, the "next big thing" is ever changing and I don't think anyone would call Rails old. It's just no longer new.