Let's see, single-player... As far as immersion goes it's pretty good, (for a low-budget fantasy wargame, which takes it down a few notches) but I found the single-player of Dominions got old sort of fast. There's plenty to discover, but the game itself sort of gets samey when playing against the AI, though I seem to be in a minority that think so. It also has a distinct lack of user-friendliness for multi-player, if you're counting that. (Though I do love the multi-player) As for how hard it is to learn, the game's very complicated in nature, but I found it really intuitive once I finished the tutorial. Even to this day, I'm not sure why.
If it helps, you can download the demo
here.