I started using Fireworks since CS4. At first I didn't understand how it was different than say Photoshop or Illustrator for web design. Then as soon as I started mocking and designing UIs for actual web applications, or applications, Fireworks showed it's strength. (Heck I sometimes use it to make logos, http://www.ptizo.com/media/preview/brand/ptizo-logo-alpha.pn...)

The number one reason I prefer Fireworks it's that the file itself can be viewed as normal PNG (and still retain editable features). The second is the mix of vector/pixel objects (all you want from vectors but as pixels).

If you have to design for the web I would recommend you to use Fireworks over Photoshop as your design tool (please stop with the shadows and gradient madness, they make the page rendering slow and scrolling crappy).

Firework's preference for objects over layers is what is key here. It drastically increases my productivity and allows graphics to be much more mutable than Photoshop.

Fireworks continues to be underrated...