I've used it to change a head from one body to another - needed a YouTube tutorial to show me how to do though. It was easy after I'd seen that but I wouldn't have been able to work how to do it without it.
It's a perfectly acceptable and free alternative to something like Photoshop, if you know how to use it you can do pretty much all the things you can in Photoshop if you know how to use that!
Installing it on your computer won't suddenly make you a photo manipulation genius though, just like installing Photoshop won't either..
It's a tool and if you don't know how to use the tool you will get crap results.