Another possible option besides gimp is the spin off cinepaint. More targeted for the film industry, cinepaint supports a more extensive set of image formats. The Gimp project isn't very sensitive to the needs of their 'high end' users unfortunately, and they aren't interested in incorporating patches for those items either.
Gimp really bogs down with anything over about 50MB and we regularly work on 350-500MB files. Gimp doesn't support high dynamic range or even 16bit images very well. Even Photoshop7's support for 16bit is poor. And color management in Gimp is nonexistent. (bummer for me) Pantone colors would be nice too.
Now, you mentioned you need CMYK support, out of curiosity... why? If you are using a windows box to control your printing system, I believe you will find, upon detailed research, that windows *always* converts print out put to RGB space before it sends it out over the wire. Most printing RIP systems actually convert to RGB internally before they compute the final gammas for each of it's target inks. So unless you are using a kodak medium format digital back with direct CMYK output (or better), Mac OS, and a really awesome printing system (Heidelberg perhaps? aka a CMYK RIP); in other words, a CMYK work flow, I'm not sure why you would want CMYK in your graphic editor.
But, even more puzzling, why didn't you ask about CMS support? Without color management, I really can't think of any reason to bother with CMYK, even with a CMYK work flow.
Sorry for being nosy but I really love color and all it's intricacies and beauty.
Oooh, I just checked the latest gimp 1.3 and it is linked against liblcms1, so maybe they do have or are at least working on color management... interesting.
PS7 doesn't support HSV either. I need to get PS8. -- Eeew yucky, they have an activation system like Quark now. stupid. You know, I actually own a copy of quark express, but we haven't bothered to reactivate it after upgrading the os on that machine. Its kind of amazing that we have a thousand dollars sitting around idle because *I* can't get myself to call them up on the phone *again* to reactivate it. I really don't like activation systems. Humm, maybe you are right, adobe is going in an evil direction... Weren't they the ones that got that Russian cryptographer thrown in jail for discovering a bogus encryption method, even more infamous because it was adobe that was the one being ripped off... stupid.
long live open source