Yeah upscaling looks fine for DVDs, and it's true some Blurays look like shit anyway. I got Daft Punk's Interstella 5555 for my birthday and it looks like they transferred it from VHS.
I only rebuy stuff if it's one of my absolute favorites and there's some kind of cool bonus on the Bluray or the DVD I have isn't working properly (like my Back to the Future Trilogy).