I just recently purchased a 5D classic in November, and I was coming from a Rebel, and I can confirm that there is a huge improvement in image quality. Even with regards to ISO, the 5d looks much better than my t3i at higher ISO's so I guess ISO quality depends on what you are comparing it to. The hardest part to get used to was all the settings and switching everything around, because the UI is definitely dated, so there is a bit of a learning curve in figuring out how to quickly adjust all the different settings.
I also had the 50mm f1.4 and 17-40 f4 to work with, so I got some great results, and I definitely noticed a richness and depth that I wasn't getting before. It was definitely an improvement in IQ, just a more polished look and better colors than my t3i.
I also would say that $450 is a pretty good deal for a 5d classic that is in good shape. When I purchased mine, I was watching ebay and they seemed to be going from the mid-$500's and up depending on how they were packaged, and who was selling the camera. B&H and Adorama were selling used 5d's for the $600 to $700 range.
One caveat is that it is heavier and bigger than my rebel, and it doesn't do video. So I've kept my rebel as a backup body to use when I want a lighter/smaller camera or for video.