Only had a brief play so far and it strikes me as one of those 'take time to understand everything' programs.
As does Photoshop of course, ok the basic image adjustments such as brightness, contrast, hue etc are reasonably obvious, but things that cut, blur, sharpen and the use of all the different brush modes less so. The best thing to do is to beg, borrow or steal a copy 'Photoshop for Dummies' and spend some time reading it to give yourself an idea of what all the tools do. (they're more or less the same in GIMP)
Mike6567, I forgot to say that negatives can be, up to a point, de-dusted using a camel hair blow type lens brush, or if you're really brave, gently wiping them with a micro-cloth but try it on an unwanted one first.