Casillas is spectacular. Maybe not the best at decision making but he has improved drastically and can single handedly (or double-handedly) win consecutive matches for madrid.
Casillas easily, watching him make save after save while his defense let's him down over and over is just amazing. Madrid could easily loose by 4-5 goals in some matches if it's not for him (most recently Sevilla) Casillas > Buffon IMO. But Buffon is quality as well.