While some individual levels can be deviously frustrating, the overall difficulty of Mercury Hg is pleasantly middle-of-the-road. If you can't tackle all of the targets for a level at once, you can break the levels down into separate, more manageable runs. However, once some of these same levels enter into the Challenge and Bonus modes, the difficulty shoots through the roof as you try to grab 23 out of 24 bonuses in under three minutes with 100% of your starting mercury intact over the course of four levels (or something to that effect, you get the idea).
One thing that doesn't help is the camera movement in the game. While you can rotate the camera side-to-side fairly freely, you only seem to be able to zoom in and out via a weird curved camera track, which isn't helpful when you want to zoom in AND be directly above your blob. There's one level in particular (I think it was Iodine... Oh, did I mention the levels are laid out in the form of the periodic table of elements?) where you have two blobs that move through very high-walled mazes with holes in the ground, but you can't get a good angle to see the blobs and the layout of the maze at the same time.
The game itself costs 400MSP ($5), which is an absolutely fair price for a game like this. There are two expansion packs of 30 levels each available for 240MSP ($3) apiece, but they don't add too much to the game. A lot of expansion levels are rehashes of the gameplay ideas from the original levels. You'll feel like you've played the exact same level before, but not quite. Still, the basic package is well worth it on its own (unless you're an achievements chaser like myself), and I'd give it a strong recommendation. If you fancy a clever take on a classic game of skill, give Mercury Hg a shot.