Jmetal fans are a silly lot...they go from casuals to those with a healthy obsession (me) to those who should probably seek therapy. The Babymetal fan pages are especially disturbing, I left all of those.
On the other side though, the fan bases are also pretty amazing, I'd say the Band Maid fans are easily the best out there of any band... They line up early for shows, and it's a day long tailgate with drinks, food, people passing out fan made merchandise, and general comraderie of fans who know each other very well through the fan pages. They'll buy extra tickets and VIP passes and pass them out to those in need (one dude gave my brother a VIP upgrade no questions asked, $200).