If you do a lot of web-based graphics, you'll probably find good use for it. Otherwise just stick with Photoshop.
Back when I used Fireworks MX in school, it seemed like something you really only used for web graphics... and it did pretty good at it. Nothing too complex, but also some features not readily available in Photoshop (vector graphics editing, image rollover effects, etc.).
Of course, as said above, a big part of it is also just personal preference.
Disclaimer: I haven't used Fireworks for at least three years, so it may have changed a bit since then.