I think it could go either way. I'm afraid it will be the Devin Townsend complex where the open tuning results in there not being a single memorable riff through the whole damn album. He does do a good job at producing a huge wall of sound, but I'd definitely take Opeth's token riffing over some wall of sound with no memorable riffs.
As for other bands that use open tunings, the only two I can think of offhand that do it OFTEN are THIRD EYE BLIND and DAYS OF THE NEW. Days of the New's second album is just layers and layers of acoustics and really sounds awesome, if Opeth does what they did with open tunings it will be an incredible album indeed.
I guess alice in chains did it on a few songs too (over now, etc).