I'm still in the process of sorting out my thoughts on the movie - but I can go ahead and give it a full and unreserved recommendation.



In the meantime, if you want to read a review, check out here and here.

Really good (spoiler-free) review here.


UPDATE
My review:

For a movie named Serenity, it offers very little calm and peace. From the get go, the story moves at a fast pace, and you find yourself transfixed to the screen, strapped into a cinematic rollercoaster.

The Firefly series revolved around the crew of the spaceship Serenity, and their adventures in getting paid. As "freelance cargo transporters" and "salvage recoverers", they often find themselves on the outskirts of inhabitied space, avoiding the law and dealing with less-than-civilized sorts.

Enhancing their troubles is the fact that two of the crew are especially wanted felons: Simon & River. Simon is an exceptional doctor who gave up everything to rescue his sister River from an Alliance (gov't) school brainwashing/experimentation camp. Throughout the series, there have been hints as to what effects Alliance experimentation has done to River - which are all revealed in the movie.

The movie deals with what is perhaps the penultimate adventure for the crew of the Serenity, and manages to bring the series story arc to an impressive and climactic ending. Serenity is in no way a two-hour long episode of Firefly - instead, it feels like an ending, or a series finale. Think of it as a Return of the Jedi for the Firefly series.

There is not much I can tell about the movie without revealing some major plot spoilers - so I am forced to comment on the incidentals.

Story - Writer/Creator/Director Joss Whedon's strength has always been in his dialogue - and it is on full display. He does more characterization in a single one-liner than other movies do in 10 minutes. The characters are engaging, the dialogue zippy and smart, and the story is strong throughout. If I had to fault it for one thing, that would be that some of the relationships of some of the main characters are either ignored or glossed over - i'm guessing for the sake of time. For newcomers, it may not be immediately apparent how Shepard Book related to the crew, or that Wash and Zoe are married. But those are ultimately nitpicks.

Acting - As usual, the cast does a great job of making their characters believable, likeable, and real. Fans of the series will reconnect immediately, and newcomers won't be far behind. The incidental characters also do a superb job (namely The Operative).

Sci-fi - Hardcore fans of sci-fi may be somewhat disappointed in this movie. Yes, it's in the future, there are massive spaceships and new technology, but the technology is not delved into with full detail - the sci-fi is there as a setting, a background, a plot device - there are no instances of "reversing the polarity", or "switching the microconverter couplers" rubbish. Whedon doesn't let sci-fi get in the way of a good story. However, bonus sci-fi points for not having any sound in space!

Special effects - The series had a few special effects, but it was never for very long - but with a movie comes a bigger budget, and it is clearly evident on screen. There are hoversleds, ion clouds, epic space battles, holographic images - all rendered beautifully.


In conclusion,

To the fans: you must see this movie. It feels as the series did, yet more intense in every regard. It resolves several very nagging questions, has many major plot points, and will induce much shock and awe. Be prepared to go through many emotions during the course of the film, and to leave the theatre unable to say more than "holy cow", many times over.

To the newcomers: you will enjoy this movie. Or else.
But really, the director does a superb (artful, even) job of introducing the characters and giving some background information so that the newcomers can catch up, but does so in such a skillful way that the knowledgeable fans are not bored from going over the same ground again. It's a movie that has many appealing qualities: action, humor, explosions, sci-fi, guns, kung-fu, spaceships, conspiracies, etc. Although much of the dramatic impact will be lost from not having seen the series, the movie still stands quite well on its own - and I imagine many newcomers will be inclined to pick up the DVD set after watching the movie.