The Sony x50 series is very nice indeed. The aesthetics is just great (casing and software) and the device is sturdy. The firmware remains the same*. Dictionaries are great. Not limited to the x50 series, but there's no need to have an account to use the ereader (except for ADE for DRMed ebooks).
However, I truly believe that without PRS+ they would lack many things in comparison to newer ereaders**. For one thing, there is 5 font sizes embedded (from xs to xl) and that's all. Many times I found it hard to read on the small size, but the medium was way too big. Usually, I was using one size during the day, and another one at night. There is no choice for the fonts, so if something isn't working the ebook needs to be edited and send again.
After maybe 300 highlights and with less than 15 books on it, my 650 became sluggish. It took more than one minute to register a bookmark or a new highlight.
In short, the x50 series is very nice, not perfect, and I think that Sony never made something as good since then (they dropped the alphabetical slider in the library
). I like to have one to complement my Kobo ereaders.
* Sometimes unfortunately, see T1 and T2.
** It's not fair to compare something made in 2010 to something made in 2013, but many people are still buying them, using them and also referring to them, so I don't feel out of line.