I saw Serenity last night, it definitely left me wanting more. I hope the series comes back... but even if it doesn'tn Joss has presented a great set of episodes and a movie behind.
I recommend seeing this movie. Seeing the series before hand (it's 14 episodes are on dvd) definitely adds to the appreciation of the characters, but I think a newcomer can enjoy the film as well.
I love these characters!