What started as somewhat progressive black metal has evolved into a full throttle progressive "retro" rock with only slight (black) metal influences. And personally that evolution is fine by me.
Epic, long songs with lots of dynamics and changes in mood and atmosphere, Opeth is a band you really have to listen to get the most out of it. It's not your average party music that you just play in the background.
Personal highlight: Deliverance (from the album Deliverance)
Other recommendations: Silhouette, Serenity Painted Death, Demons Of The Fall, Face of Melinda, The Drapery Falls, A Fair Judgement, The Grand Conjuration, The Lotus Eater, Windowpane