I've used Awesome exclusively at work for about 6 months, and I want to insist a bit more on the tag system. I've come to believe that Awesome's tag system combined with tiling mode provides unequaled power and allows setups that KWM or Metacity can't do.
For example, I have two terminals with tag 1 ("term") and two editor windows with tag 2 ("editor"). That way, I can quickly switch between terminals and editors. So far, nothing special compared to workspaces.
However I've also tagged one terminal and one editor with tag 5, and the others with tag 6, that way when I'm in a terminal and want to check code, I can easily (and seemlessly! no flickering) replace the other terminal with an editor by pressing Win-5 or 6.
I also have a mail client and web browser with their own tags (3 "mail" and 4 "web"), and can easily squeeze them in to the current view by doing Ctrl-Win-3 or 4.
This demonstrates both multiple tagging a given window and displaying windows from multiple tags. It takes some getting used to, Awesome is definitely a "power-user" tool, but is completely worth it.