I agree this game is underrated because of Lumines, but I like it much more, certainly as a single player game. Very pretty, very clever, balanced, never unfair, each level is a new landscape with a different challenge. It gives you that whole wide-eyed, "oh my god, look at this level" feeling later on. Surely one of the best games available for the psp.
While Lumines is very nice, I tend to get bored of it as games just go on and on forever or until you die. I prefer the goal-oriented level design of Mercury, perfect for hand-held gaming. Just lovely.