I disagree. I don't think they were underrated or overrated. I think they got just as much mainstream attention as they deserved, but I would have to say that what you get to hear on the radio is by no way their best material.
I'm a huge fan. Not so much of their really early work like the Screaming Life/FOPP EP or Ultramega OK. But Badmotorfinger, Superunknown and Down on the Upside are all three extremely loved albums.
An underrated band, in my opinion, would be something along the lines of Mr. Bungle, Dysrhythmia, Behold... the Arctopus,Frank Zappa and the Mothers of Invention or Frank Zappa in general.
One of the most overrated banRAB ever is probably Led Zeppelin, if only for the fact that the two most talented merabers of the band are the ones who were largely over-looked.