Pretty amusing and original concept (at least for me - mind you I never played Dungeon Keeper, to which everyone is comparing Dungeons against). Nice graphics and some good jokes. The strategic element is really good - you really need to plan ahead, so strategic thinkers would be pleased to give it a try.
It wears out too quickly however. The missions are very repetitive, you do exactly the same over and over again, mission after mission. And maybe I would live with that if it wasn't for the scenery, which never changes, like NEVER...
I was able to endure it for about 4 or 5 missions - until unlocking all the gameplay elements (which are gradually introduced to you mission after mission). Then it got too boring and I dropped it.
If it was just 5 missions long, I would give it 7 - 7,5. But it's too long for the level of variety it offers...