Disclaimer: I haven't used the most recent version of GIMP yet. Also, I'm not a designer and that colors my position.
What I've found to be the case in prior versions is that it's not sufficient for many minor edits, because it's not 100.0% compatible with the PSD file formats. Even 99% isn't enough in some cases, unfortunately. Since I'm typically editing a file created by a designer in Photoshop, and since I might need the designer to build on my changes, I don't trust anything less than 100.0% compatibility.
Isn't that a separate issue? It sounds like it is sufficient for the edits, but not for the task of working with other people who use / content made in Photoshop.
You're right that in some sense it is a separate issue. I could do my edits in GIMP some of the time. However, since I have to keep Photoshop installed for interoperability, there's no real reason to also install GIMP.
What I've found to be the case in prior versions is that it's not sufficient for many minor edits, because it's not 100.0% compatible with the PSD file formats. Even 99% isn't enough in some cases, unfortunately. Since I'm typically editing a file created by a designer in Photoshop, and since I might need the designer to build on my changes, I don't trust anything less than 100.0% compatibility.