This largely depends on what you're planning to do. If you plan on doing a lot of vector work, then I'd say it's a tool you should consider. There is, of course, Illustrator which is slightly more advanced for vector work, but it doesn't have the good file compression options.
And then there are other, cheaper alternatives, so I'd say, download the trial version of the various tools and see which one you find to be the most intuitive.
If you just need to do the occasional raster graphics and already have Photoshop, then you don't need it.
I'd not buy Fireworks just for its good PNG compression abilities. For that there might be tools that probably do the job just as well, e.g. like the tools in the article I posted or using PHP's GD Library.