There are a lot of long songs in the metal world, but I end up liking only one out of every ten. Most of the time these bands create a song that's long, but only because there's a lot of filler.
For me, Theocracy's 20-minute song on the latest album, is a perfect example of a song being long because it has a story to tell. The story is so dense that there are no long instrumental breaks. I was thinking of naming that as the best epic song ever, but even though it's long, it doesn't really strike me as an epic. But it is probably the coolest long song ever.
Even a lot of good bands are guilty of just having pointless instrumental interludes, or even interludes without instruments at all, but just spoken word and some keyboard flourishes to make it seem more dramatic! Nightwish's Poet and the Pendulum, as good as it is, just didn't need that middle spoken part that lasts for 1:30 or so. Avantasia's Seven Angels also has a pointless interlude of about 1:00-1:30 in the middle. and then there's the wanking that goes on in some of the more progressive epics. Dream Theater is really guilty of some of that. Although some people dig that, so I can at least understand that they are appealing to a core audience.