Like several others, I use Manga Studio/Clip Studio Paint and love it; I use it pretty much exclusively now, but I used GIMP for a number of things in past. For years, GIMP was my main graphics program because it did what I needed it to do -- for example, its batch functionality saved me so much tedious work and time. There were just things that other programs couldn't do, or you'd have to pay for another program that you'd only use for one thing -- GIMP's free and fairly easy to learn, even if it isn't the absolute best program out there.
Unfortunately, the development is not always responsive to problems and suggestions, and well...it's free, so you can't really expect it to operate like a program that's had a fully-funded development, testing, and maintenance. As mentioned, there are certain issues with the OSX versions that don't generally occur when using it on a Windows machine, but then I haven't done that since 7 a few years ago, so I can't speak for its functionality right now.
I'd advise people starting out to at least have it available, because it can do plenty of things that other programs can't do, or don't do as easily and accessibly. Overall, though, I'd always recommend Manga Studio/Clip Studio Paint for anyone creating art, especially if they're creating comics.
