I'm a photographer making the switch from Mac to Linux, and currently trying to make GIMP do the work. Alas, there's a serious problem that runs deep: the colour/value processing functionality does not work as it should. You can prove this for yourself with 3 easy experiments:
Open any photo of a decent size and duplicate it into a new layer. Zoom in to 300% or 400% to better see the effects. Desaturate this new layer using the Hue-Saturation command. Set the layer's blending, transfer mode to Value. You won't fail to notice that the desaturation has seriously affected the Value, something that should not happen. The value should have remained the same.
Revert the image to its original and again duplicate it into a new layer. Now we'll approach the problem from the other end and pretend we want to reduce colour noise by blurring the colours only, leaving luma Values the same. So apply a strong blur to the new layer and change the blending mode to Color. Again, the luminance is seriously affected, giving the image an appearance of a greater degree of mottled grain. This should not have happened, the luminance should have remained the same, only the colours should have become smoother and perhaps more muted.
To satisfy ourselves that this is not merely a problem with layers, we tackle the problem in yet another way. Revert the image again, but this time don't duplicate into a new layer, apply a strong blur to the image directly. Now pull down the Fade command from the Edit menu and set the transfer mode to Color, leaving the percentage at 100%. It's no better, the luminance is as badly affected as it was with the layer method.
This leads me to my question: is there an easy way to report this as a bug? I've looked at the Gimp Bugzilla page and can't make sense of it. Would someone experienced here care to repeat the tests and do the report? We could treat it as sharing the workload...