I love the dictionary on my Sony 650! It's so easy to use, I'll use it for words that I'm pretty sure I know the definition of.
I upgraded from my 505 to a nook specifically to get a reader with a dictionary, but was very disappointed. I found the dictionary on the nook to be very cumbersome to use and on top of that a lot of times it didn't have the word I was looking up in it.
So, even though I'd only had my nook for a couple months when the new Sony's came out, I upgraded again. And as I said earlier, I'm loving the dictionary support on the Sony--so easy to double tap words to get definitions of them!
dordale