I have Photoshop on my work laptop and I use it mainly out of convenience but don't enjoy it. I have actually tried to get all assets as SVG and stay in Inkscape if at all possible.
Inkscape's UI isn't the best either , I'm not quite sure why I prefer it to Photoshop but I do.
So it's the same UI it's had for years. Just because it isn't modern and pretty doesn't mean it's a clusterfuck. I personally find Gimp easier to get started with than Photoshop, for one...
There have actually been some major improvements, notably single window mode. I believe they recently fixed the thing where the Tab key (Show / Hide Docks) does something unexpected if the last thing you clicked was a toolbox icon. These days it seems like the Gimp devs are a bit more tolerant of user feedback, which is one of the reasons I'll be making a small donation.
> Just because it isn't modern and pretty
The problem is not that it isn't modern and pretty.
The problems, at least for me, are stupid blockages that Gimp tends to put in the way of getting things done, eg. the layer boundaries thing, or the fact the font box keeps the focus after you've selected a font. If you need to work efficiently these things quickly add up to major annoyance.
Pinta [1] is a simpler alternative to GIMP on Ubuntu. I never used it (and where is the source code?) but it looks similar to how I remember MS Paint. GP was asking about Macs, sorry I don't know anything about them.
In any case, the value of UX isn't just about how well experienced users can use it, it's also about how well new users can learn. Blaming a tool for skewing to far towards the experienced users is a valid form of criticism. Moreover, it is easy to have the skew without noticing. After all, developers likely move in circles with many more experienced users. There is also the potential blowback from experienced users to any kind of change.
I miss Macromedia Fireworks, if anybody recalls that...