Yeah, for most things, GIMP user interface is absolutely fine.
Most of my mild grumbles about GIMP are not really usability problems, rather just features that would be nice to have. (We got better layer groups now, yesss. Non-destructive plugins are finally within the viewing distance. Yay.) There is stuff that GIMP doesn’t do but competing open source apps do, and while there’s interoperability, it could always be better. (I hear GIMP is finally going to come up with a replacement for xcf files, hope they cooperate with Krita devs and blow the dust off the OpenRaster project.)
The only really aggravating GIMP usability thing is that exporting is very janky (would rather have one single unified export dialog with all the settings), but I assume that’s just a thing because of the way GIMP file plugins have worked in the past.