I just upgraded my 505 for a 350 a week ago (with plans to give my 505 to my mom for Christmas), and I would say it's definitely worth it, if you've got the money. When you compare the two side by side, the text is so much easier to read, its lighter, smaller, easier to turn pages, and the instant dictionary, which was the only thing that made me even think about a Kindle when I was shopping around two years ago, is great.
If you're missing out on the sale, and cost is a factor, have you thought about the 350? I originally was going to get the 650 because I thought it would be good to have the memory card slots - I've never gotten around to using them on my 505, but I always wished it held more books, so I assumed the 650 would be the better bet...but they have 1.4GB of internal memory now, which is 1,000+ books. So I opted for the 350 since I don't think I'll ever need to have that many books on me at one time (or even own that many).
The 350 is smaller than the 650, though the store I went to play with one didn't have a 650 so I have no idea how much of a difference it really is, but going from the 505 to the 350 I haven't noticed the smaller screen size. I have noticed how nice and small it is overall, and how I can comfortably hold it in one hand, and how much clearer it is to read, but haven't noticed the screen size.
Anyway, I'm happy with it. Upgrade away!
