Firefly deserved an ending, and Serenity tried to deliver. If only Joss Whedon got one more season, the whole idea of Serenity would have got more time to develop the plot and wrap up in a non-hurried manner. As it did not, Serenity is the best we could hope for in the circumstances.
It ends the story, kills off a couple of characters and solves the mystery of the origins of the most terrifying peoples of the whole 'verse. It provides suspense, nicely choreographed fight scenes featuring River, good insight into the characters' personalities and some good old fashioned drama.