Not really that evil.
From a technical standpoint, it's as impressive as an XBOX 360 game should be. The generic voice acting shows its limits pretty early on, as minions say the same things over and over, but it's never annoying and it's always effective.
The Overlord looks extremely awesome, but his poses don't quite match up. It's hard to describe. His poses aren't nearly dramatic as they're probably supposed to be. He often looks non-threatening, like perhaps he has to go to the bathroom. He probably would've looked a lot more awesome if he had a flowing cape.
The Overlord gets a very small amount of movement. He can't jump and has limited places that his own feet can take him. One button attacks (a limited number of animations for essenitally the same attack) and another button casts a spell.
The real joy comes from a button that sends his minions forward, and the right joystick which controls all of the minions to make them go wherever the player wants. Later in the game, the player apparently has to control more than one group of minions, but that's not in the demo.
From what I did play, the "evil" Overlord was awfully heroic. After slaughtering some helpless sheep (or more specifically making his minions do it), his first actual task is to kill a bunch of halflings who took over a farm. The farmer himself is left unharmed (in fact he can't be harmed at all) and is grateful for his farm being rescued from the halflings and their "talking" pumpkins.
Next, the player comes up to a raised drawbridge guarded by some humans, who tell the Overlord that they'll lower the drawbridge if he goes and frees the villagers who are held prisoner by the halfings. The halflings at this point are obviously more evil than the Overlord. Perhaps the Overlord is just getting rid of competition so that anyone who's working is working for him.
The game explains that killing innocent lives brings corruption to the land (which is good!), but letting people live is supposed to be good, too, or something. Realistically, the only way to be an evil Overlord is if there are people to rule over, so that's probably the approach it takes.
After killing the evil halflings and their evil troll boss, all of the peasants are freed by the wonderful and heroic evil Overlord and the demo ends.
For being a badguy, the Overlord is awfully friendly and plays the role of the savior.
I read that the player's minions are strictly limited to tiny goblins of varying colors. This disappointed me, because I'd been hoping that the minions would develop into bigger and more impressive creatures, but the goblins are still pretty great.
There's no co-op or multiplayer, which is as always disappointing, but for people who need something fun to do by themselves (besides the hairy palms path), Overlord seems like one of those games that would be really fun from beginning to end.