opeth unplugged strikes me as entirely unnecessary - their whole songwriting dynamic is based on a combination of heavy and mellow, they can perform amazing mellow moments but thats' it, mellow moments.
then again, i guess if they decide they want to do it, i think it could be something really special.
other unplugged sessions, i have a soft spot for nirvana, alice were always amazing. and semi acoustic, the gathering were very impressive.