My biggest issue with Demo is the empower spell. It makes the spec feel very clunky. Summon cooldown demons, empower, summon dogs, empower, summon imps, empower. Empower, empower, empower. It's very boring.
Mechanically, the spec is either purely single target with some minor splash AE or mediocre burst AE that sacrifices a bunch of ST damage. Maybe Demo ends up a big dick single target meter topper, but mechanically it's still a ramp up ST spec. That's just not all that valuable for anything but the two patchwerk fights we'll get the whole expansion, or boss fights without AE phases that are on farm.
Locks are pretty much forced to go Destro for progression right now. Destro will remain one of if not the best two target cleave spec in the entire game. It's also really good a priority burst damage which is important during progression for obvious reasons. That toolkit has it's place. Demo isn't anywhere near that.
Still though, and the reason I'm switching to Mage, is that with the exception of a fight like Twins where there are two targets and only two targets 100% of the time, Fire does everything better and is more mobile.
Demo visually is awesome though. From a class fantasy perspective they nailed it.