But I wouldn't advise you to try out the demo, the demo wasn't representative anymore for what

really became in the end.
It would still give a feel for the game, though, even if it was based on code a few months before release, and is a little heavy on combat. After loosing interest in Diablo, just the few quests in Aleroth and the character / item / world interaction was enough to make me want the game.
Actually the main problem with the demo is the fairly large file size (400MB) considering you would probably not want to finish the demo and then re-do the beginning catacombs in the full game right away.
The Beyond Divinity demo is smaller (and shorter), but starting in a prison there is not much for quests or character interaction. The combat system it fairly similar to DD, other than having two characters (stat and skill systems are different between the two games).