Been a huge Clutch fan for years...their sound has changed a bit over the years (which is great), and they've always delivered. If you have a chance to see them live, do it - intense show. Their drummer, JP Gaster, is amazing and has a groove that won't quit. Dan Maines (bassist) has the best job in music.
Their past two albums (Beale Street and Strange Cousins) are mainly blues-based rock, and their earlier stuff (Transnational, Clutch, Elephant Riders, Pitchfork) was really intense. The stuff in-between (Slow Hole to China, Robot Hive, Pure Rock Fury, Blast Tyrant, Jam Room, etc.) is just some of the greatest music you've never heard. Get the lyrics sheet, and an encyclopedia - Neil Fallon writes some killer stuff.
Good clean fun. enjoy.