I have a Vaio laptop which I upgraded from XP to Vista - Core Duo 1.66, 1.5GB RAM, crappy Intel graphics. It was always slow on Vista - I blamed the rubbish graphics and HD - so tonight I was gonna wipe it and put XP back on. Then I remembered that I'd acquired the 7 beta the other week and thought 'what the hell, might as well give it a go!'
So far, I'm really impressed. Installed quickly, installed drivers for all the components that Vista couldn't. The differences are definitely not just cosmetic - my laptop feels like a different computer. Where the Vista install was always laggy and hammered the slow HD, the 7 beta feels nearly as smooth and snappy around Windows as my Q6600 with 4GB on Vista. This is what Vista should have been.
I can't see there being many delays for Windows 7. This beta seems very stable and pleasant to use.
(edit) Been playing a bit more. Feature-wise, it's very similar to Vista apart from the new taskbar. But I'll be getting it because on slightly aging hardware, it seems to me to be faster than XP, let alone Vista.