My very first dedicated ebook reader was the Sony Reader PRS-505 in purple that I still have in my collection because the hardware is still fantastic. Sony's Readers were always the most expensive, but they also were quite innovative and always had premium hardware. Thus, it's a bit sad to write that their Reader Store is closing on 20 March 2014.
I purchased several ebook readers over the years, I still use a Kindle Paperwhite for long periods of reading, but spend most of my time reading on an iPad today. The dedicated ebook reader market has slowed down quite a bit from the days when Sony, Barnes & Noble, Kobo, and Amazon battled for consumers. The leader is clearly the Amazon Kindle brand and it is tough for me to recommend any other today because of risks such as we see here with Sony closing down the store.
The good thing is that Sony is not leaving you hanging and will be transitioning content to Kobo. For the next month you can re-download your Reader content and save it for readering. In late March they will send customers an email with a link to transfer their existing Sony library to Kobo. There may be a few titles that are not in the Kobo store so your safest bet is download all of your library titles and back them up for future use.
Sony put together a helpful FAQ page that should answer all of your questions about this transition. I have books at both Sony and Kobo and am actually looking forward to having one centralized library for all of this content. I imagine the Sony Reader iOS app will also be removed when there is no longer a Sony Reader Store to access.