That's exactly how I use it as well.

What I'd like to see is a "★/☆" for a repo, branch, or commit. Similar to the way you can ★ a question or answer on StackOverflow.

I definitely do this. If I'm actually "watching" a repo, such as tenderlove's ARel repo, I'll put the RSS feed into my reader. Might be a little clunky to do it that way, but that's basically how I get all my other news.