it's part of the 'midweek madness' special so the offer is valid until Thursday only. i remember trying the demo when it came out but not sure what to make of it.
Of course! After you point this out the Steam home page updates to show a big ad on the front page about it. (c:
I tried the demo and it seemed fun enough for $5. I can honestly say I enjoyed the Trine demo more than I've enjoyed the full version of Torchlight.