Activating Steam Guard is a must nowadays, not just in the context of issues like this but in general. With Steam being as popular as it is, it’s reasonable to assume that it’s going to be regularly targeted by security attacks and incidents like this are going to become even more prominent in the future.
Hopefully Valve were able to resolve everything on the backend before the issue spread out of control, but it would appear that a lot of damage has already been done. Users are strongly displeased with the situation, and a quick glance at popular gaming forums is enough to confirm that.
The main issue that people seem to have is not so much with the fact that the hack happened though – after all, we’ve learned to accept that nobody is safe from security exploits – it’s in the way Valve have been communicating with their users about the issue.
They are known to be relatively silent and secretive in general, but in an incident like this, sending out a mass e-mail to warn users to keep their accounts in check should be one of the first actions they take. The situation is still developing, so affected users should follow the news and social media for information over the next few days.