On Saturday morning I bought and installed Minecraft, mainly to see what all the fuss was about.
I finished about 11pm...seriously dangerous game (and apparently there is another update up yesterday). The basic premise is that you are in a fully exploitable world formed of blocks, and initially need to build a shelter at night, to keep the nocturnal creatures away. You start with nothing, and you'll probably need a little bit of background reading to find your feet. I quickly attacked some trees, fashioned a workbench, and then a wooden pickaxe, before hacking out a small cave to live in, and a door to keep the beasts out... After that you'll start hunting some of the various ores, and before you know it you have a 4-storey castle (complete with crenellations, and in my case a glass-bottomed balcony, to allow me to check the door from safety), with a fully functioning mine complex in the basement. You'll start channelling rivers into your domain, and stream lava from nearby mountains.
I sort of forgot there were zombies outside for a few hours, and went about lighting my main storage areas with glass lava-filled pipes. I'm now on a hunt for volumes of iron, as I want to build a mine cart track between my main digging pit and the enormous tower I'm building with the residue. The zombies turn from a threat into a resource, as they drop various items that are of use (string, feathers, meat, and one particuarly nasty breed drops gunpowder).
So yeah, playing it is bad for you...it'll be worse once the developer (a single guy) gets multiplayer working properly, then a bunch of people could attack a world, and probably hollow it out. I did have an attempt at setting up a server, however the PC kept dying once someone connected for too long (might be the PC, it's the one I picked up off Fish, but with an old install of XP from ages ago...)