I upgraded from a 350 and I love mine. The Wifi is extra useful, as I can download my paper (I subscribe online and I get a link to a PDF on my paper's website) without having to plug it in every day.
It's very fast; the interface is so fast actually that I often forget it's e-ink and start using it as if it were a tablet.
The browser, being based on the standard Android browser, has
great support. In landscape mode, I can play interactive fiction with
Parchment.
I would have preferred it (or at least the official case) to have a slot to keep a stylus, but it's not too much of a problem as I've started carrying a capacitive stylus (with a rounded metal end that works great on IR touchscreens) for my tablet and phone anyway, so it's not like I need to keep Sony's piece of plastic clipped on somewhere.
With it running Android, I can't wait to see what our hardware hacker friends will be able to enable on this. Just thinking of an offline Interactive Fiction player gets me very excited.