I actually saw the forums were down, and was like, man, I'd better start up a game and make sure it still works. Turns out Shatter actually started. And then darnit, I ended up playing like half an hour.
Curse you Steam for actually working!
This is the ideal way to do it, but I'm pretty sure that if you say, start up your computer with no internet connection, you can launch in offline mode even if you were in online mode previously. Also, if your internet connection goes out while Steam is up, you should be able to continue using it without issues (which you can test by pulling the Ethernet cable).
I think some people DO have problems with it, and there's like one specific sequence where you will get screwed over, and some games can decide they needed to be updated and prevent you from playing them, and there may be some mystery offline mode expiration where you can't do it for more than however many days in a row, but I don't know the details if so. I agree that it has problems and should be better, but it's fairly decent.