Right now. Go.
However, there are a couple of caveats. There are only 30,000 keys available and there's a good chance they'll all be gone soon... seeing as the site is struggling to stay online under the weight of demand. Still, try your luck.
I only got round to trying Rise Of The Triad recently and wish I hadn't waited so long. It's one of the purest 'old-school' shooters in years; bringing the action back in its twitchy slippery glory rather than trying to change it for a new audience. I much preferreed it to Wolfenstein: The New Order if I'm honest.