Ok, I've seen two such films in a row. I am Legend No Country For Old Men. Both films build up the story and characters really well, and then choke right at the end. NCFOMen even won 4 or 5 Oscars! WTF!? Are bad endings considered cool/chic these days?
I haven't seen NCFOM yet, but I agree on I Am Legend. It began to spiral a little around the middle but came back, only to crash completely in the end.
I am legend is really a disappointment in the ending department. Another bad ending would be matrix revolutions.