I think they're the only band whose best album is their greatest hits (except maybe Queen). None of their albums sounds like one piece of work. None of them have a specific sound. If I hear a QOTSA, Tool, Mastadon or Raconteurs song I can instantly tell which album it's from by the overall sound but Foo Fighters albums have no theme or specific sound.
They are a great rock band and I have a lot of respect for him.