We finally got around to watching the NetFlix-sent "I Am Legend" a few nights ago.
I think the best analogy is watching a mile race. An underdog runner comes from nowhere to lead the pack by a large distance, running at a pace that would be a new world record. In the middle of the final lap, however, the runner slows, dramatically, and then the other runners begin to pass the early leader, and finally the leader stops running and walks into the stands and eats a hot dog, never finishing the race.
85% of a great movie is still a bad movie if you paste an "and it all worked out after all" at the end.
Of course, we didn't watch the alternative ending (didn't even think of doing so) that was apparently on the DVD. I did watch it online yesterday, and am stunned that the producers didn't select it instead. Much, much better.