You haven't seen Serenity yet? What are you doing reading this blog? Get out in the big blue room with the evil day star (heh - been meaning to insert those trite little things somewhere) and go blow a few bucks on celluloid. It's big action, romance, comedy, political, science fiction.
It's certainly not a summer big-n-stupid opens-before-memorial-day blockbuster such as Deep Impact (Flight Plan beat it in the weekend box office, for Jeebus' sake), but it's not a make-you-think science fictioner such as Gattaca or Eternal Sunshine of the Spotless Mind.
Now mind you, I occasionally enjoy them big and stupid. Matrix almost fell out of that category until you saw the sequels and realized they had no idea what they were doing. The Core is very, very, very stupid, but very, very, very, very fun and never takes itself seriously. Stanley Tucci's pompousness alone makes it worth a watch. Unobtanium? Sure, why not.
If you've seen Joss Whedon's other recent works, Angel and Buffy, you know he likes to do nasty things to his characters, and they certainly don't have an easy time of it here. Far too little is used of characters outside of Mal and River. The best lines by Jayne and Wash are in the previews ("I don't want to explode" and "Define interesting." - "Oh god, oh god, we're all gonna die?" respectively) but certainly not all the funny stuff, and certainly not all the action.
Reavers! Reavers and more Reavers! Mr. Universe (no, not Ahnuld).
If you don't believe me, read Orson Scot Card's review.