North Korea, that crazy old uncle of China and South Korea tested it's first nuclear weapon... Now when other countries test nukes it's always a little like them wondering if fire is still hot. North Korea on the other hand doesn't have the most stellar record getting it's missiles to work. The Tae Po Dong tests for example conducted recently were spectacular failures mostly breaking up over the ocean.
So it's little wonder really that they feel the need to test them. Should the world really be that worried though. South Korea which hasn't been crippled by sanctions or a crazy old uncle in charge (that's you Kim Il Jong in case you're reading :P) took many, many years to get building a car right. I'm sure we all remember the first Hyundai Excels that were "tested" on the world. Some would argue that even now they're only just starting to catch up. So if it takes 15 years for the smarter country to get something as simple as a car right (or at least passable) how long will it take the crazy, dog food eating, cardigan general?
Even if they do manage to get it to 'work' (by which I mean not blowing up in a silo, launchpad or over one of their own cities) what would they want to do with it? They might think having a crack at South Korea is a good idea... but that's a pretty close target so the fallout would undoubtedly affect the North as well (probably doing billions of dollars in improvements however). What about Japan? Those two have never really seen eye to eye but they are a little further away and to get there they'd probably have to fire it over China or at least Chinese air space. Even a crazy like Kim would have to know that pissing off a country with a massive (bored) standing army, nukes of their own and a billion people who live just across a line in the jungle from you might not be a good idea... China had no trouble at all getting a lot of soldiers into Korea in the 50s to help the North so I'm sure they still know the way to the border.
Of course Nth Korea isn't the only country with a nutter in charge. There's a religious zealot in this far off land of the United States of America who has "nucular" weapons all of his own. So perhaps what it might come to is two crazy uncles sitting on their porches with big guns trying to shoot garden gnomes off each others porches. In that case I'm not sure who I'd pick as the winner. Uncle George has more weapons and they work but he's not quite as crazy as Uncle Kim which could count against him.