Photoshop has a nice looking UI but there are some very subtle but really useful things GIMP does that Photoshop doesn't. For example when drawing with the Rectangle/Ellipse tool GIMP doesn't immediately lock in the selection but gives me handles I can use to perfectly fine tune the selection down to the pixel, especially for the ellipse tool. I don't get that level of control over the marquee tool in Photoshop.

I can also freely scroll and zoom while using the lasso tool in GIMP and it can go between free draw lasso and poly draw lasso by clicking and dragging or just clicking to establish new lasso nodes. And I can just undo any of those nodes by hitting backspace.

The only tool I really miss in GIMP is the magnetic lasso but needing to drag the cursor all the way to the edge of the screen just to scroll somewhere else on an image makes the tool so much worse.