I once played Haven in my common room back at school and some of the more open minded people (although not metal fans at all) said it was amazing stuff... I said I thought this music would be more appreciated generally if the vocals weren't so devastating and they said the vocals suited it and it wouldnt be right without them... This coming from people who very very rarely hear screamed vocals. I was very surprised...
The first time low growls are heard by anyone I think it's very hard to appreciate, and I told them I was impressed they could appreciate it. They said, (this is a direct quote) "It's impossible not to appreciate this." (unenthusiastically I might add. They weren't into it at all, but that's just taste.)
The same people winced at intestine ballism, Rabies Caste, Dies Irae, Dew Scented and Kalmah on other occasions.
Basically, I think the general opinion of informed metal heads is that Dark Tranquillity are pure genius and one of the more important metal bands in the history of extreme music. The un-metal public won't criticise it, and if they do they'll still be able to see "for a shouty deathmetal band", it's of a very high standard.