It's really sad that Overture didn't receive too positive rankings, not because it's too difficult, but because the learning curve is crazy. I own this game and still play it often (no online, though), and I have to admit that it could have been better if they had more characters from the main series included, but that aside, Overture shares one great similarity with the main series: the awesome and highly unpredictable story, which confused and amazed me at the same time.
One other point for the poor reviews is because almost nothing about Overture is anything like mainstream stuff: no real fighter, no real RTS and especially not too easygoing on newbies in any of the genres.
Still, this game deserves more attention than it has: or have you ever watched a replay match between 2 pro gamers? Way more exciting than some BB or GG replays.