The girls seem to be in the nascent stages of blowing up globally. I'm seeing more and more posts about BABYMETAL in my FB feed from friends who've never listened to JPop. The cool thing is that, while it's clearly picked up the past week, it has actually been happening on a smaller scale for about six months now lead by actual metal fans who legitimately like them, and now the more mainstream music fans and bloggers seem to be turning them into the hot new novelty. Which will mostly pass quickly, but I think they might have more staying power than, say, Psy, based on the fact that BABYMETAL's novelty is based around the mesh of musical genres, as opposed to one specific viral video. At the very least I think they'll get a proper world tour out of this before it's all said and done.
Remember, they did have their mini-boom a while ago with Doki Doki Morning. It got picked up by a decent lot of Western music and Japan-oriented media, but it didn't really stick in the long run. And I did have similar worries about them going down Psy's way, but they seem to be regarded more as proper artists rather than a one-off gimmick (which in all fairness is rather unfair to Psy). At least people do seem to be discussing their entire discography on serious metal websites. It's rather hilarious seeing die-hard metalheads try to make sense out of them.

I looked up some stats for their channel and it sure enough was a steady growth, but the actual viral explosion only happened on and after March 1st, that is the first Budokan day:
Yeah, I think people assume that "Gimme Chocolate" is their debut or something, and most of the western media articles are heavy on "OMG! The music apocalypse is upon us! WTF Japan?! LOLZ!!!" and woefully light on actual content about the group. My favorite song is still "Ijime,Dame,Zettai," and I've been making a point to link to that video whenever someone posts about them on FB. Such an epic song and video.[/size]
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What does the fox-god say?"...

I don't really blame the mainstream media for being shallow, that's what they are. And I stopped keeping track of them, but I saw mentions of BM in nearly all major outlets over the last week. Music media do focus on their actual music content though, so that's good.
Speaking of IDZ, I think I mentioned it before and I'm still not 100% sure, but I do think we visited the site where they shot the PV when we were in Japan. It sure does look a lot like it: