I have has a Sony for over a year and I love it. I almost never pay for books as there is always something I want to read from the public library's ebook system. My local county system didn't have them this time last year (few in my state came up as compatible back then) so I drove 2 hours (one way) to spend 7 lousy minutes in another counties library to fill out a short form to get registered in their system so I could do the books online thru their system. Now my local system uses the same vendor (they got a grant to pay for it).
I have little need for color, so I am not concerned about upgrading anytime soon. If and when I do in the future it will be to the next size bigger, but the one I have fits perfectly in my purse, I use the small padded slip case it came with the protect it (less bulky in my purse and protects it great). My eyes do not tire (this is something people ask me or comment negatively on all the time??), and it works wonderfully! I feel like I am "cheating" on my love of real books...LOL...but for all those times I am sitting in Dr offices, in long lines, etc its great. For someone who travels, or maybe commutes on the bus to work, it would be great. I also am a person who likes to be reading several different books all at the same time, so this works fantastic for that weird option!
Good luck with your shopping!