I was not able to find a way to reduce noise. Also, there didn't appear to be an eyedropper tool in the hue / saturation adjustment area to pick the color tone I wanted to adjust. These are both part of my typical Photoshop workflow, so lacking these features is a pretty big deal for me. All the typical places I'd think to check to alter the resolution of an image--image properties, canvas size, etc.)--didn't allow editing.
And, it does not appear to be easy to record your own actions with GIMP. As a photographer, I rely heavily on recording my own actions in order to streamline my workflow. I understand that GIMP allows even more complex scripts to be created than Photoshop, but at the cost of ease of use. I didn't delve deeply enough to see if there is an easy way to run or convert Photoshop actions for use in GIMP.
I tried to create a simple graphic for the Web and found the text tool to be clumsy and restrictive as compared to Photoshop. It was more like using a word processing text tool than a graphics program's text tool. The inability to apply styles to more than one part of a text box and the separate text editor pop-up were not user-friendly at all. Once I created my graphic, I had such a hard time finding a way to slice and save it for the Web that I eventually just gave up. If there's a way to do this in GIMP, it isn't obvious or similar to PS.
Okay. That ends the contributing commentary from my wife. I will write the summary. GIMP is powerful. Way more powerful than your average user needs. Of course, so is Photoshop. For some, GIMP may seem a bit much--too many options, and bells and whistles. But, it has the tools to get the job done for most users.
If you're a professional photographer, it could be another story. Granted, GIMP has tools and add-ons you can use to expand its capabilities, but it is still no Photoshop. At least GIMP is free, though, compared with $700 or so for Photoshop CS5, so you get what you pay for, and for most users GIMP is probably a better bang for the buck than Photoshop.
Those aren't the only two options, though--on any platform. What do you use for photo editing? Do you have any additional recommendations for Linux or Windows? Any suggestions for a Mac OS X photo editing tool?