Makes all the sense in the world, for Sinestro.
(I'm stealing from a reviewer)
Not the Sinestro as we see him in this film. He's the best character in a goodness sense. Internal logic doesn't support him donning the ring. Some hints needed to be dropped about a character flaw, or even showing him feeling seduced by it a la Bilbo.
IIRC, you just see him putting it on. Why? So he can become evil for the sequel, is all. I liked the move in general for its not-so-pretentiousness, but the Sinestro-ring thing did need a touch of characte develpment to explain it, IMO.