Overlord is one of the best open world action adventure games I've played on the PS3. The game combines RPG and RTS elements for a rather long, well written exploration. I was pleasantly surprised with the story and missions, most open world games (anything by Rockstar) try to trick you into thinking your on a story driven mission when you're really just on a fill in the blank side mission. Overlord's story is really put together well and keeps the gamer focused on the objective, conquering the world, and not trivial missions that lead to more trivial missions. Once your horde is a good size (over 30) you can really wreck havoc with your minions and your full potential for evil is realized in humorous ways. It has some flaws, but if you like long exploration games with story driven objectives then I'd recommend buying the game. Rent the game if you're not sure.
The Flaws: The learning curve is not realized until near the end of the game. Some upgrades are based on the number of souls you've collected, each time a minion dies you lose one soul, so by the time you get near the end of the game when all the really cool stuff is available you might not have enough souls to get any higher level upgrades. So, try not to waste you whole horde like I did several times in the beginning of the game. The sound wasn't too bad, however after 30 plus hours of gameplay your minions talking really grinds. Unfortunately, you can adjust all audio settings except for your minions. The main character (Overlord) never speaks, which is something that bothers me in any game. Controlling and targeting the horde takes getting used to and could have been improved [however it's still better that Overlord II]. It's too easy to accidentally sweep all your minions into water/fire/poison or danger and then all you can do is watch them die. There's no in-game map, so you pretty much have to remember where things are.