The battery on my PRS-650 died a week before I was due to go on hoiday
I was pleasantly surprised by the price of the T2, so I took the plunge, rather than go on holiday without my reader and mess about with the battery later. I certainly wouldn't have upgraded if my 650 hadn't died, but now I'm glad it did!
First big difference is the weight - T2's plastic case vs 650's metal, so it is swings and roundabouts. You lose the benefit of the little magnets that kept the front cover closed (on the lighted cover, at least), and I'm slightly more nervous about dropping it, even though the real risk is screen damange in either case.
The second thing I noticed was the screen contrast. This does seem to vary from person to person, but my T2 certainly has better contrast than my 650 did (and I have compared them side by side to confirm that).
But the thing that really surprised me was the improved speed of page turns and screen updates generally. With my old 505 I used to start turning the page about three-quarters of the way down it; the 650 improved that significantly, but the interruption to reading caused by the T2's page turns are comparable to the time it takes me to turn paper pages (OK, so maybe I'm not the world's fastest paper handler, but you get my drift).
Another difference is the method used for attaching covers. The old "spine pins" are gone, and the T2 clips into its cover.
Finally, Sony have (finally!) got round to using the cover of the book you're reading as the display when the reader's in sleep mode. This pleases me inordinately, but may be of no significance whatsoever to others!
As I said, I wouldn't have upgraded if my 650 hadn't died, but after a month using the T2, I've got to confess I'm quite glad it did! Would I upgrade from a 650 knowing what I do now? It's a very close call and the honest answer is I don't know - if I had the price of the reader (and a new cover) burning a hole in my pocket (or had the money to buy myself a present, or whatever), then yes definitely; otherwise, I'd probably be able to find a more deserving cause for my money (mortgage, food, etc).