I think you find more real fans in the Underground, most of the "mainstream metalheads" tend to be into it then grow out of it shortly, once you explore the underground extensively you know you love the genre. I listen to more black metal than anything and I find it very hard to connect to metalheads from other genres, even the black metal fans who only listen to the big bands like: Mayhem, Burzum, Darkthrone, Dimmu Borgir, Immortal, (Though Dimmu Borgir and Immortal annoy me more than anything), while there's nothing wrong with liking that stuff, I don't feel like we will be able to connect over the music, and I find those who listen to the mainstream tend to be less passionate.