Underwood is a great dude and a great teammate. For a team looking for bargain wideouts, you could do worse. Ted Ginn is a better player though, no other way to slice it.
Underwood is serviceable working underneath and thats about as far as that goes. He isn't a catch in traffic kind of guy Good news is Cotchery can fill that role for you. Underwood is best used to stretch the field with his straight line speed. Ginn had that ability as well but what Ginn has that Underwood doesn't have is quickness and change of direction speed/acceleration. Underwood isn't anywhere near as able with the ball in hand as Ginn was. He isn't going to give you YAC's unless he is already behind the defense.
Not a bad signing but if he is anywhere above your #4 option at wideout, you need to upgrade the position more.