I had the chance to compare a 50D and a demo 5D Mk II side by side last night. I'm now a convert to full-frame.
It wasn't an in-depth comparison - just grabbed a few room shots and compared using the back panel - nor was it very scientific (they had different lenses for instance) but it did bring up two great features of the 5D Mk II:
1) The viewfinder is huge! It's like being at an IMAX cinema. I'm sure I could park my car in there.
2) The noise levels on the 5D II are incredibly low. I was snapping away at ISO 6400 for a while without any obvious sign of noise in the images. I hadn't even realised it was that high. Possibly some noise reduction was turned on (I didn't check) but even so the image detail was great. The 50D also did well for itself - ISO 800 was nice and clean compared to my 30D, ISO 1600 was low noise and 3200 wasn't bad. But the 5D II had a really noticeable win. Only when pushed into the H1/H2 (ISO 12800/25600) range did it start to look a bit yucky.
I was surprised at just how much difference there was. The 5D also sits very nicely - the 50D feels small afterwards.
That said, a disadvantage of the 5D II was apparent - shooting raw, each image is around 30Mb big. 30Mb! A typical shoot of 200 or so images would take around 6Gb to store. Compare to currently where the same shoot would fit on a single 2Gb card. It's not the kind of feature that stops you buying a camera, but it does require some infrastructure to support it. A half-terabyte drive now only stores 83 shoots and that's before you start using Photoshop.
That said, I'm thinking my next purchase will still be a 50D. I'd love a 5D Mk II, really would, just for those clean images, but it's still an extra £1000 over the 50D. And £1000 buys speedlites, lenses and a few days of studio hire. And travel. And I really need to go to Reykjavik, Iceland and take some models out into the countryside. (As will hopefully be explained later. :) )