I went this weekend, rather than waiting for a discount show later in the week (as some others have said they'd do) because I think the film needs the opening-weekend numbers. The success or failure of a movie (and the question of whether sequels will be funded) is based upon one thing only: Opening weekend box office gross. If you wait for Tuesday, you might as well not go.
I went not as a fan of the Firefly series. I haven't watched it yet (though Matt's DVD set sits on the table.) I wanted to see if the film stood up on its own without knowing its backstory. (If it can't do that, you can't recommend it to other people.) It works, and it works well. Yes, there are certainly series details which would make for a deeper appreciation of the film, but there was enough exposition given quickly enough that you Get It.
I went tonight not as a fan of Joss's work. I haven't even watched Buffy, save the studio-mangled film. (Which I love, though I'm full aware the studio mangled his vision of it badly.) I went purely on the strength of the opinions of people I trust. My friends generally don't steer me wrong, and today was no exception.
So go Sunday. The Sox have clinched, the Cubs and Bears are done, you can TiVo everything else.