Have you ever played ICO, or are you just saying that because "everyone else" is saying the opposite?
ICO has its own fanbase, and within that fanbase it's appreciated, but its sales were never anything spectacular, and so, to the general video gaming audience, it's safe to say that it is "underrated."
If we're not going to specify our demographic, than you could make the case that any game is overly, or under appreciated.