It's good. Go see it.
Finally a big splashy science fiction movie with writing and good dialogue. Some of it may be confusing at first for people who haven't seen the original series (it's good. Go rent it.)
It's great to watch a Joss Whedon production in a theatre full of excited people - what has until now been a mostly private experience, watching things on TV, is suddenly a communal experience. It's good to hear a whole room erupt in laughter - although the witty follow-up lines are often lost, which I guess means I'll have to see it again.