I've been using GIMP 2.8.2 for about a year (just upgrading to 2.8.10). I used to use Photoshop for work about a decade ago so I know some of what I'm doing.
For basic image editing and vignetting etc. it's good. I know there is a lot more to it but I've not really spent the time. There are tonnes of "how to..." guides online so if you don't want to pay for PS or the others it's worth a go.