I love Nightwish, have done for a long while now. *wears badges*
'Stargazers' is probably one of my favourite songs by them. 'Oceanborn' and 'Once' are my favourite albums by them.
Yeah, Nightwish certainly aren't underground, if they were underground, they couldn't fill huge arena like they have done.
The band decided that Tarja should leave the band, as she was more focused on her solo career than the band or something. I've read previously that she doesn't actually like the metal-esque sound of Nightwish. :/
Here's an article from Kerrang! about Tarja being sacked, which I posted on my LiveJournal.
http://img401.imageshack.us/img401/9272 ... 0537ke.jpg
Oh and I love Lacuna Coil a lot more than Nightwish, I've been championing Lacuna Coil for years now. But I don't think you can compare Nightwish and Lacuna Coil really, seeing as Nightwish have that operatic aspect to their music.