Underrated:
Pendulum- I don't think they've had a bad song yet, and although a lot of people may recognise their music, they tend not to know that the song is by Pendulum
Hovercraft Pirates- Small band from somewhere around the north of Ireland, and I really doubt they'll ever be known [certainly not well] from anywhere outside of there, Ireland or the UK, but they're really good and have some really good songs. Although their songs tend either to be really good or really bad, not much middle ground.
Overrated:
Oasis- although I like Noel and Liam, so many of their songs sound the same, so if one's not very good, most of them aren't.
McFly- and not just from a "damn those pansy boys!" type of thing, but I just think they really suck and they're really annoying. Of course, very few people over 13 actually like them, but a few years ago them [and Busted] were a sore spot with me...damn pansy boys.