For the two very most common bitmap editing tasks(color remapping and selection-based edits) it's OK and it even boasts some fancy features these days. For drawing, it has a workable set of brushes and fills, though I think Krita is the better app here now. For graphic design things(typography, layout, layers of procedural effects) too many things still involve a slow, indirect workflow, but there are signs of this problem being chipped away at.
(The single best tip I have for GIMP is: get in the habit of using the "/" key quick search instead of using the dropdown menus)
This one is more obvious maybe: I also found learning the keyboard shortcuts really useful when working on a web comic.
I don't have anything but praise about level of thought that went into modelling the UI and considering different use-cases, by author(s) himself. Extremely open and inclusive (in the best sense of the word). These people know what they're doing and do it well.
I was a user when the Krita team focused on painting. I admit that I was among those disappointing at that time because I naively wanted Krita to become open source Photoshop. I thought they were making a mistake in the direction they took. I'm happy I was wrong about that.