I prefer full-screen, because (1) those stupid black bars annoy me, and (2) I'm not really missing out on anything anyway.
Firstly, I think that the annoying thing about widescreen isn't about black bars at all. It's the fact that widescreen has a wider aspect ratio than 99% of TV's, and as a result the image on the screen is "smaller". It can be harder to see than a fullscreen image, which is "closer" and "bigger" and easier to see. If you have a 20" TV, and you're sitting 10 feet away from the screen, it can strain your eyes to see a widescreen picture. But, here's the solution: just sit closer to the TV!!! It was meant to be seen close up, and it's better like that anyway.
Secondly, "not really missing out on anything"? On an epic movie such as Spider-Man 3? Look at it this estimate:
An artistically composed and beautifully excecuted image. But, if it were fullscreen...
...not only is that beautiful image being cropped and essentailly trashed, but Venom would end up being in the movie even less than he was before!
And it's not just Spider-Man. Imagine Transformers in fullscreen:
being trashed to:
Imagine all the detail you would lose in a scene like one of Optimus' transformations. Filmmakers (and effects artists) put that much detail into the entire picture of the movie for a reason. The entire image is created with great care, and like
kaijunexus said, having that detail cut out is like having a book cut off on the edges. Those edges are important to the work of art as a whole.
Essentially, I think viewing a movie in the original aspect ratio is vital to experiencing a film. We're meant to watch the movie as the director intended. If he wanted us to see the movie in fullscreen, it would have been filmed that way.
Appreciating the aspect ratio not only respects the artist, but yourself as a viewer, providing them both the proper attitude and satisfaction.