There's always a constant debate and many thoughts and chats shared over there regarding what's better for your career, to be a generalist or a specialist. It's not an easy one, and I tend to take the position of having some of both as your seniority increases. Very similar to what it's proposed by Scott Ambler in "Generalizing Specialist".

In my humble opinion, junior devs should start specializing in a topic and trying to develop themselves in a generalizing way, because the more you knew broadly the more you are able to peek at another specialization that could really suit you better. Then you can remain there for a while, but, turning you at some point into a senior developer, could force you to continuously broaden your knowledge if the team/company/startup you are could benefit directly from that.

I've found some discussions about it from time to time on Twitter, so I created a Tweetdeck collection which occasionally I update.

Generalist vs Specialist at twitter