Opeth live absolutely kicked my arse, ripped out my my entrails, shoved 'em back in with a pitchfork and repeated. In a good way!
After the gig (and after I listen to any of their albums), I feel uplifted and happy. I feel like I kick butt.
When Virgin Black played (they were supporting, just before Opeth came on), that was when i felt like shooting myself. Dark, trudgy, gloomy songs with no 'hopeful' sounding parts, similar to My Dying Bride. After listening to them I feel like removing my own entrails.
I think that the lyrics of Opeth could be depressing if they were shoved along with some sorta MDB-esque gothic song. But with the music Opeth plays, the songs become a huge variety of emotion - sometimes depressing, sometimes uplifting, sometimes just plain ANGRY AND BRUTAL AND TR00.
And like some people have said here, I too have trouble listening to My Dying Bride for long periods of time. I once had a nap and had The Dreadful Hours playing. After a wacky dream involving some really angry bald dude, some place burning down and a baby half burnt to death, I woke up very, very depressed. Heh.