I have never owned the DVD version of Fistful so it was good to have it in high definition. I must admit it was a letdown in picture and sound quality. Sounds weak, colours bleached plus some horrible graininess.
Again never owned the DVD version of Few Dollars. This suffers from the same problems as Fistful. Grainy and quite flickery too. Sound was good though. If you're happy with your DVD versions, stick with those.
I have however owned several versions of GBU and this is superb. Excellent picture quality and clarity of sound. No grain (that I noticed at least) and even the extended scenes look good.
GBU is worth upgrading for fans, but I would imagine the upscaled versions of the DVD of Fistful and Few Dollars would be as good as this bluray effort, if not better.