All the intel agencies are saying it was a nuclear test. Russia is saying it was between 5 and 15 kilotons.
South Korea and the US have both cited .5kt, Japan hasn't made up its mind whether it's real but seems to be leaning towards the smaller figure, and China hasn't commented on the yield yet.
The size of the weapon worries me. I'd feel safer if it were 15kt. There's less probablity they'd be able to deliever it successfully that way. .5kt is small enough to put on an unguided artillery rocket...unless the bomb fizzled. As long as no actual high-order detonation actually occurs there's still a chance that when Mr. Il inveitably has a temper tantrum and chucks a nuke at the ROK that it won't provoke a nuclear reaction from the US or anyone else.
Seoul has a real chance of stopping a tactical ballistic inbound. They bought a lot of Patriot PAC-3s for just that purpose, and the missile was designed and has proved capable of doing it. An unguided FROG-7 or BM-series rocket on a low trajectory is another matter.
As to why North Korea shouldn't be in the nuclear club: well...they're loonies. It's unfortunate to have to use that wording, perhaps, but there's really no other way to describe them. North Korea has repeatedly proved to be the least tractable and most unstable country in the world. Mr. Il has repeatedly proved he doesn't give a **** about pissing off anyone in any fashion, and it's not clear he even understands the concept of consequences. He has repeatedly attempted to provoke his neighbors even when his neighbors are the only thing keeping his country fed. MAD always assumed the other guy was rational in some vague sense. Kim-Jong Il may not be.
And it's a hell of a thing to have to explain to the citizens of South Korean we let their country get nuked on the basis of fairness. The gun metaphor is nice, but it's also flawed: you can't afford to act on an innocent until proven guilty basis with something this dangerous.