I have two boys, an 8yr old and a 6yr old. My 8yr old is pretty much a Minecraft nut. I paid $26 for the PC ver directly from mojang.com. They also have it on the xbox360, and on the iPad. But the PC ver gets to see the most use. Actually last week, i had to cough up for a 2nd copy, so that my 6 yr old could join into the online worlds that the 8yr old has been obsessing over. He plays online, downloads all the maps, and asks me every other day about getting him a mod. He also loves watching the youtubes and being part of the minecraft community.
What I like about it, is its teaching him the PC quite well, and he's now a whizz on the keyboard. He's been doing his own resource packs, and downloading maps. So getting a good idea of pc directory structure.
I have NO IDEA, on what exactly what the attraction is. I've asked, and left confused. All i know is that they love it, and it seems pretty harmless.