I've played an Alpha of Overlord for about 30 minutes and it looks and plays quite cool. The official voice-overs were not in yet, so instead of those you got some of the devs doing some voiceacting which was aweful but funny.
The humor of the game is refreshing after what most developers nowadays concider funny. It has some political incorrectness to it.
The art is very good I think, and the gameplay is refreshing as well. It plays a bit like dungeon keeper but with a tad more "personal" feel because you actually control the "dungeon lord" aka Overlord.
The controls were a bit messy, but then again that could be me because I played it on an XBox 360 and I never play consoles. You used one of the direction buttons for controlling the Overlord and the other for controlling the minions in a group or seperately. Pathfinding issues were still numorous in the Alpha, but I expect most of those to be fixed in the final release.
It's cool to see all those minions harass a farmer and destroy his fields
In the beginning it was all about learning the controls, harvesting souls etc to be able to summon more minions. First task was to get the Heart of the Tower back, which was in the possession of some farmer. Basically Fed-ex, but different because you have your minions doing it for you.
Kind of micro-strategy meets adventure with slight RPG elements. Of course your choices will be somewhat limited because you always play an evil character. But as Triumph stated, you'll be choosing exactly how deep your depravety goes :D
In short: I'm looking forward to it as a break from my rpgs, but I hope the controls on the PC will be sensible.