well I can tell you I like it!
But is definately not one of the run of the mill kinda games. Hell even 360voice classifys it as an oddball game.
But to answer question no it is not like oblivion, yes the are both RPGS (Well sorta) but either/or wouldnt come in mind when playing. If you have played and enjoyed Pikmin 1 or 2 then this definately a title to pick up. Overlord is basicly Pikmin only all grown up :P
But yeah you probably should get overlord (In my opinion anyway) If you are still in doubt about the game you should download the demo from the marketplace as it gives you a little taste as to what you are getting into.
Oh, and if you decide to buy the game you should probably get it soon. well I mean before the second one comes out anway...