While I think Gimp is a great application, I would never use it on a Mac. I have it on both my SGI and Sun systems, and Gimp is way better than Photoshop 3.0.1 (the newest version of Photoshop for my SGI).
I really think that X11 has been an easy out for some developers. It has given them the ability to ignore the Mac and still say "will it does run on Macs". Yeah, it runs on Macs, but then again so do Windows apps in VirtualPC or Parallels... but that doesn't make those Mac apps.
When looking at open source projects I see a number of different levels of commitment. You have both Gimp and OpenOffice which basically would just as soon spit on the Mac as look at it (neither SeaShore nor NeoOffice are part of the main development communities of Gimp and OpenOffice). And then you have projects like AbiSource and Mozilla who make an honest attempt at making Mac versions of their apps.
While neither AbiWord nor Firefox are going to replace TextEdit and OmniWeb for me, I consider those two communities to be putting the Mac platform on an equal footing with other platforms they support. But I would rather pay for Photoshop or TIFFany3 Professional (both of which I own), or use free apps like ToyViewer and PixelNhance (both of which I use), than use Gimp in X11.
I have said that I felt that both AbiWord and Firefox are only as good as they are on the worst platform on which they run. And that rather than taking advantage of what makes the Mac great, they follow a least common denominator development plan.
As someone who wants to see apps take full advantage of Mac OS X, I've said the same thing about most Carbon apps (Carbon apps tend to have few true advantages over their original Mac OS versions in Mac OS X). But That doesn't mean that I don't respect the fact that the least common denominator development plan puts the Mac at an equal level with all other platforms... which is way better than being an after thought like we are with both the Gimp and OpenOffice communities.
On other platforms Gimp and OpenOffice are great apps. I've been using Gimp since 1999 on my SGIs and have loved it! But running Gimp or OpenOffice in X11 on a Mac is like running Windows apps in VirtualPC/Parallels/Wine on a Mac. If you let developers off the hook by saying that those environments are good enough, more developers are going to stop making any effort to make real Mac apps.
In software development the squeaky wheel adage is really true. The Mac has survived as long as it has by Mac users not settling for less. By saying that it is okay to run Gimp and OpenOffice in X11, it is telling those communities that we are willing to settle for less... which means they'll not put any additional resources into making true Mac versions of either of those.
All I'm saying is don't reward Gimp (or OpenOffice for that matter) for their lack of efforts towards our platform of choice.