http://en.wikipedia.org/wiki/Nightwish_discography#Charts
Check out their chart positions on this page and tell me they aren't an extremely popular band.
Metal bands mentioned in this thread on Last.fm:
Nightwish - 504,783 listeners
Cradle of Filth - 264,661 listeners
Opeth - 272,471 listeners
Dimmu Borgir - 227,324 listeners
Within Temptation - 329,779 listeners
Mainstream "metal" bands on Last.fm:
Slipknot - 528,203 listeners
Mudvayne - 272,196 listeners
Static-X - 299,444 listeners
Lamb of God - 216,165 listeners
Killswitch Engage - 299,147 listeners
You can also look up the chart positions for any of their albums and I'd wager that each band has consistently been in the charts with their last few albums and singles, so I can't see how they aren't mainstream.