Ever since I first listened to Opeth (February of 2006), I've had a love-hate relationship with them. I think Opeth's first two albums "Orchid" and "Morningrise" are good prog-metal albums with some neat melodic death metal touches, but until their recent switch to straight-up prog-rock (which is actually pretty decent), they've meandered in this wishy-washy hybrid of prog-rock and melodic death metal that didn't jell. Too many of Opeth's material made past "Morningrise" felt too dreary and grey-sounding for something that's supposed to be metal, as there's been quite a few times where I actually felt a tad sleepy listening to such songs.
If you want to check out Opeth, get the albums "Orchid," "Morningrise," and "Heritage." Beyond those, you're better off looking into the prime works of Cynic, Gordian Knot, Atheist, Nocturnus, Phantasmagory, Pestilence, Death, Coroner, Watchtower, Dream Theater, Rush, Symphony X, Fates Warning, Behold the Arctopus, Blotted Science, Spastic Ink, and Atrocity for metal bands with progressive ideas that still play with some sort of energy and overall superior songwriting.