Yeah, my Sony 500 is history now. The basic reading experience is very similar (the screen is probably identical to the 505 in terms of size and contrast, and slightly better contrast and faster speed than the 500), but the reader software is much better (the user interface, buttons, and the navigation system).
I would say that in terms of the screen, reading text off it, and the physical size and weight of the device, it's 10% better than the Sony.
In terms of the reader software, navigation, etc. it might be twice as good.
In terms of the overall experience, the free integrated net access, the Amazon-quality store with 4x the available titles, web browsing, subscriptions, and instant-gratification book delivery without the need to sync with a PC, it might be ten times better.
That extra $100 is absolutely worth it, and I cannot imagine buying the current Sony device at this point when the Kindle is available.
I still can't say if this is a revolution in ebooks or not, but it's way better than the Sony in my opinion.