I didn't think demo damage was viable either at first, but if you line your abilities up and use lifeblood effectively, it's pretty good.
Lifeblood -> corruption -> double hand of guldan -> meta -> sbolt spam (with imp fast casting for 100s) bursts people down pretty quickly.
It is also the most mobile spec by far, being able to leap every 10sec, and has more utility than other specs.
The more I think about it, all three lock specs should be pretty good.
After an expansion of getting camped by multiple rogues, i sense it could be time for a bit of payback, hope there as good as you make them sound. If that doesnt work out i'll just play any healing class