In my experience, Fireworks is far less useful when it comes to the creation of graphics after the initial wireframing. On the other hand, Photoshop [and GIMP] is completely usable throughout the entire design process. For this reason, many designers would rather learn, and pay for, only one product, and that product is Photoshop. As a side note, although GIMP lags in a few areas, notably on the typography side of things, it's the best product for my needs when I weigh cost and features, and only rarely do I find myself wishing I had tossed several hundred dollars for Photoshop.