Casillas IMO is great indeed. He's been at the top since he was 18, played many CL finals, came off the bench to save Real from defeat in the CL final once (cant remember against who tho). He has great positioning and reflexes, he's excellent in shot stopping and one on ones, altho he could work on his handling a bit.
Sttill, a quality keeper tho, I rate him very highly, in the top 3 in the world.
As for the most underrated, I would have to go with Lampard. Sure, we all know he's a great player, but noone talks about him when it's award time. Everyone talks about Henry, or Rooney, or Gerrard, or Ruud, etc, but IMO Lampard is by far the best player playing in the Premiership.