I think Bitbucket is pretty good, and has some advantages--most notably that I can have private repositories there without paying. Yes, this may identify me as a cheapskate, but, well.

If anything, I suspect this investment is to make Github more competitive with Bitbucket -- remember, BB's parent company is Atlassian, which has pretty deep enterprise experience they can leverage. You may like GH's issue system better than BB's free one, but a paying BB customer can integrate it seamlessly with Jira and other Atlassian tools.

I do use Bitbucket and it does exactly what I need: track changes to my private source code repos and manage issues.

Does GitHub do some things better than Bitbucket? Sure, but I don't believe that those differences justify a valuation approaching a billion dollars.

Indeed. I use both and have to say that there's nothing killer about both of them. I have my private projects on bitbucket, but feature wise they are interchangeable.

I do honestly prefer Unfuddle.