Overrated: All shredders. SHredding is not at all that difficult and a lot of kids get a WOW(God, I hate that expression) when the hear someone playin with 200 kmph. Personaly, I dont' find it musicala at all.
Underrated: Guys who dont shred for some reason. Lately, however it appears that more and more of guitarists whom I have thought of as nonshredders, as guys who were never able to play fast solos, turn out that they can doit. Examples, Mike Einziger(Incubus) solos his ass off in one song of Incubus' 1st album. The same goes with Tool's guitarplayer -- in one song in the 1st album he goes running up and down the fretboard.
I don't want to start again the epic war between the shredders and the nonshredders but I had to make this point.