I don't know if underappreciated is exactly the right word. He's certainly much better known than any of the examples listed thus far, and I think the general reception to him is positive. However, when he's mentioned, it's often about his voice, his lines. Those aren't bad things to be known for, I guess, as they are fairly entertaining. More derivatively, a lot of posters think less of him because he chooses to serve Talia. I think that's kind of bullshit, since it draws an interesting parallel between Batman's ally's thematically.
But not many people talk about how he felt like a huge threat throughout the movie to Batman. I remember in the theater watching "how the hell are they all gonna come back from the shit he does here". Granted, there were some plot hiccups through the movie that undermine that aspect in retrospect, but still, I thought he put on a great show.