As a Photoshop teacher and Gimp book author (french reader can have a look at http://www.le-radar.com) i'm interesting in comparison because this is a very good source of idea. But i'm a bit fed up reading gimp cannot do anything as good as PS. Most of time it is possible, but just have to a way to, because in my point of view GIMP is DIFFERENT and don't have be a PS clone. But there are few things i can say :

A) I'm not sure i've understood everything but some told few days ago, that it was possible to do in GIMP (with layer modes) all what Adobe PS does in layer effects. At this time the best thing i know for gimp is the layer_effects.scm. But may be I mistake.

Even compared to this script, PS's functionnalities seem to be much more comfortable : the effects are updated when the original layer is modified.

B) Photoshop has a very interesting functionnality in being able to group layers (pasting in new can be done by copy/paste and layer->new).

C) The add of several layers increases much more the size of a XCF file than it does with a PSD

D) It is impossible (may be it is a technical reason) to work with transform tools on a masked layer. In my mind, the tool could be easily applied to both, but i'm sure i mistake.

We could tell much more about this but may be all this is not the more important. I guess that :
- working in other color spaces (CMYK, Lab ...), with specific channel options [including working directly on channels allowing copy/paste from one channel to another ...] not only for DTP purpose but because some things can easily be done when there is a specific access to a luminosity/black channel.
- improve the more and more the (anti)alias : in some cases pixel selection or transition still produce ugly effects.

In conclusion, no tool can be perfect. Loving Gimp, i have to confess that i spend much more time doing a thing in GIMP that I should with PS. But we also can find defaults in PS : there's no copy visible (you need duplicate and flatten), less filters, no real script developement because PS's Script is just a way to pack already-there-actions, no capacity to sharp a softened selection ...

So those who don't believe in the Gimp could also take notice of this.

cedric