I bought the full game about a month ago. It's well worth buying. The content will last for dozens of hours (plus the devs recently released new campaigns) and some bloody hard difficulty levels too that'll make you really think how to do the mission.
The one gripe I'd have is that the missions gets very samey - every one I've played thus far (perhaps about 15) has involved simply having to capture certain cities within a turn limit, with no variety like having to defend. However there are loads of units to help keep things fresh.
If you're loving the demo then you should dive in with the full game.