Aha! If you've been fretting about whether XCOM Enemy Unknown is the second coming or an act of outrageous vandalism, now you can - partially - find out for yourself. A two-mission plus base-browsing demo has just arrived on Steam, which sounds like it's primarily tutorially but should give you a flavour of Firaxis' 21st-century reimagining of the classic 90s TBS. Get it from the link on the right of this page. Be warned it's a chubby old six gigabytes, however.
The download process is a bit squiffy here at the moment, so I don't know if the demo is fully live yet, but 2K claim it "will be available today", so keep trying. I'll update as and when it works properly here. Update: it works properly here! So there.
Also, there's some of batshit bonus incentives thingy unlocking new stuff as more and more people pre-order the game. We already know about the Guile 'do, there's also a bunch of TF2 customisations and now they're promising a free copy of Civ V to all pre-orderers if enough extra people pre-pay. It's currently 47% of the way there. 47% of what, I don't know. But it seems likely it'll pull it off before the game's release on October 12.
I don't know as yet (due to those download problems) whether this demo is an accurate reflection of the full game, but I do hope so - and that you like it as much as I do.