Fluid design - truly fluid - is hard. Very hard. It's not just a question of page width - do your fonts scale, and does everything scale with them? Ideally:
- Sizes should be defined in
em
rather than px
- ...and that goes for element sizes, not just fonts!
- Given a change in font size or zoom level, the page elements should be the same size relative to each other
Our main product is fluid, and it's a pain from my point of view as a designer, especially because it involves a lot of user-generated content.
For one thing, images - in a fixed-width site, you can have an image that fills half the width, and looks great. In a fluid site, this image is just as likely to be lost in a sea of whitespace, looking rather lonely.
Life should be easier once border-radius
and other CSS3 properties come into play more, but sadly our core audience are government workers, who all, ALL STILL USE IE F@!*ING 6!
To answer the question, "is it worth it"? Yes, if you do it right.
Here's a scenario: choose a fixed-width site: your boss displays it to a client on his brand-new, 1920x1600 laptop, then complains to you about "how it all looks small on this guy's screen!"