I've used both, and Gimp falls flat in the implementation
    of the features that it does have.

I've also used both, regularly and for many years, and my experience is the opposite. As I keep saying, Gimp's UI and portability is what makes me keep using Gimp despite also having Adobe CC installed on my workstation.

    And that's where most open source software completely
    falls down, because getting UI and polish into an app
    takes tons of time and experimentation to get right.

I could just as easily say that this is where proprietary software falls down because software vendors sell their wares on features, not polish. As a professional software engineer, I know this first hand. PS continues to get feature after feature but its basic rectangle select tool is still a complete pain to use. Last time I checked, the Gimp team flat out refused to add support for 3D rendering. It took them along time to come round to the idea of adding vectors.

Actually, if you look at Gimp's development history, you will find that their approach for a long time now has been to work very hard on making the features they deliver as powerful and usable as possible.

This is partly why features are added so slowly. The other reason is because the development team are a handful of people who work on the tool in their spare time.

Yet despite this, they have built an excellent image manipulation tool in Gimp itself, they've developed the GTK+ UI toolkit that is the backbone of the excellent Gnome and XFCE desktop environments and countless applications. They've also developed the excellent GEGL library, the non-destructive rendering engine built precisely to enable adjustment layers (or their equivalent) in Gimp that you can't live without, which in the meantime has enabled the rapid development of Darktable, a very capable alternative to Lightroom.

In a futile attempt to keep this conversation on topic, I'll just point out that both GEGL and GTK are licensed under the LGPL, while Gimp and Darktable are both licensed under the GPL.

    Just because you don't find a feature important...

That's got nothing to do with it. I started working with Gimp because it was the only tool that ran on Linux that was capable enough to do what I required.

A user of a tool like Gimp or Photoshop makes the application work for him. Before adjustment layers existed, Photoshop users duplicated layers and blended them with the original to get the flexibility they required. I do the same in Gimp. As Gimp introduces more powerful features, I will take advantage of them.