I've played against enough Rammus to say that I don't think he's op at all. Mind you, may be because Warwick just doesn't give a fuck about tanks, but even then I still play other characters and never think him any more an inconvenience than any other tank.
I've been thinking about what mtr said about Warwick being a mediocre carry, and it's not something I've ever noticed. If I'm doing Warwick correctly, then I've eaten their carry early and often, gotten Dragon on timer, and solo'd Baron. At that point, I'm firmly in "just don't give a fuck" territory, even farting around the enemy base while my team is halfway across the map. The good and bad thing about Warwick is that it's more about control and momentum with him then any other character. For instance, a bad game I played not too long ago I went 4-8. Why? Because their Anivia was taking not just their Golem buff, but invading to get mine. And you might think, so what, no one can go 1 on 1 with Warwick, but Rammus was with her literally everywhere she went. And I might not give a fuck about Rammus, but I wasn't going to 2v1 before even level 6, and by mid game Anivia and Rammus had swayed control from me. It took two players to outjungle Warwick, but without that control and momentum I was just fucked.