I'm a fan of Jeff Bridges so I snapped this up on DVD without having seen it previously. I instantly became a fan of Kevin Spacey, also.
The general plot of "alien in human body" has, in my experience, been done to death, but I liked the approach taken in K-PAX, which struck me as quite different and much more sensitive than most. Bridges makes an excellent doctor (you *want* to tell him what's on your mind) and Spacey's expressions and gestures are perfect in the role.
The interaction between Prot and the entirely human characters in the clinic is delightful, particularly the subtle manipulation of their behaviour.
The ending is bittersweet - I like Prot's choice, but the final scenes (with Bridges) made me teary.