I liked Firefly. I really liked Serenity (even if it wrecked the damn Reaver myths dammit). It may be the best thing Joss Whedon ever did, as Buffy went stale for me around Season 3 and got progressively worse, while Angel was always fantastic but started to fade away around Season 5 (though if it had got a Season Six would've been so epic). Firefly never got the chance to go stale because it was cancelled way before it's time. And Serenity was good but I always hoped it'd be the first in a series of movies. Apparently it won't be, which is sad, as the cast are genuinely talented and doubtless have lots of good acting gigs on the go (especially Fillion, aka Mal).
I was in Page 45 in Nottingham (finally remember Dr. Sordid posting it was the "grown-up" shop ages ago and went for a looksie) and I saw a graphic novel of Serenity - Those Left Behind. The original Dark Horse issues went like hotcakes (which makes me wonder just how unprofitable Serenity was given it's comics are worth a mint), and semi-amusingly even Nathan Fillion (Mal) had problems buying issues for his family without being fleeced by some city slicker.
The storyline is set bang between the last episode of the series (one of the weaker ones IMHO) and the movie. It ends with Inara getting off the ship and Book deciding to do the same. I believe I read somewhere that Joss wanted to do an Animatrix type prequel to Serenity The Movie but didn't get funding, and that this was the script (or more likely synopsis the other authors worked with). It certainly does resolve one of the plot-threads from the TV series, the Hands of Blue - who were frequently seen chasing our heroes and killing people with their weird devices but never actually caught up with them in the 13 episodes run.
The comic didn't work for me. It's not bad per-se and I recommend you read it if you like Firefly/Serenity. It was only 3 issues long, and to me needed to be a six issue mini-series.
Maybe Serenity needs to be on a TV to work for me, maybe the plot was too close to the movie (it opens with a raid on a bank, the middle is in a spaceship graveyard and so on). The witty banter was there - but again I think I needed to hear it, not read it.
It could also be the main reason I bought it was to finally see what those god-damn-hands-of-blue fellas were about, and after 2 issues of buildup they last roughly five seconds against Simon, River, Book, Kaylee and Wash, without any revelation of what made them so special, and without them remembering to use their funky nosebleed devices. They basically die hastily so that the Alliance can call the Operative and begin the movie.
The comic also goes to the bother of digging up Lawrence Dobson from the first episode (he was the Alliance agent shot in the eye and apparently if the show had gone on for more than 13 episodes would've popped up pissed at being maimed by Mal - I had to look this up, it's been a while) only to have him decisively killed of by Mal in slightly more time than it takes the secondary characters to kill the Hands of Blue.
So the upshot - buy this book if you're a completist (it's going between the Firefly boxed set once I err... buy it and my Serenity DVD).
Also Wikipedia reckons Dark Horse are doing another miniseries - one that sees the crew of Serenity pulling off a successful job which results in unexpected wealth called Serenity: Better Days. I'll buy the trade no doubt when it comes out. Hopefully it'll be a little better.