Mercury, although a great concept at the start, becomes overly annoying, and very frustrating... very fast.
Mercury sees you tilting the level to get your little blob of... what else?... Mercury, to the end of the level. To do that, you might need to split your Mercury into two parts to open a door and sneak through, or you might have to change the colour of your blob to get into a certain area - anyhow, it might seem easy at first, but after a while, you're going to get ridiculously frustrated, even more so than you would playing Stuntman - and that's quite something.
Mercury, although it doesn't appeal to me very much, might appeal to a PSP owner who likes puzzle games like Lumines or Exit.