For just reading e-books, I like the built in dictionary and the speed of the page turns. When I went to the Sony store to look at the readers, I noticed the 600 seemed faster, but it didn't seem like it would make much of a difference. But now that I have it, I find that I am turning pages too soon, because I was used to my 505 where I had to hit the button early to be able to read on to the next page early. I am used to it now, but it really does make a difference on the 600.
I don't have to make annotations right now, but I like the idea that I can. There have been a few times in the past when I would have liked to been able to do it, and now I can.
Also, my husband works for Cisco, and through a program they had with Sony, I got $30 off the 600.