The demo, initially due to end this week, will finish at 10am PT/1pm ET/6pm GMT on Monday, Blizzard announced on the game's forums. It's still only available to those who have purchased the $65 BlizzCon virtual ticket.
If you never bought a ticket or just don't fancy wading into the demo, which starts you at level 15 and caps you at level 19, then you can read Steven's impressions here. He called it "brutal and boring", and couldn't get used to some of the old systems—but still ended up loving it anyway.
World of Warcraft: Classic is due to launch next summer, and it'll be included in any current World of Warcraft subscription.