I like the Penta concept. My favorite is their Leipzig hotel (should you ever go, pay the premium…read moreand enjoy their lounge - it's blast. If you do not have the time to enjoy it, at least check it out). The Penta Kassel is, unfortunately, only a small-scale copy of the Leipzig extravaganza, but still worth it's 4****.
If you dine there and like your steak medium rare - forget it. Tried it umpteen times, they never got it right. Rest seemed ok, though.