Mine is a unique experience. I purchased the Sony PRS-600 for the wife for Christmas. I loved the device but hated the Sony Reader Software so much that I sent the reader itself back. I turned around and bought her the Kindle 2 to replace it. She loves the Kindle 2 but is someone who buys all the latest books and not interested in many of the older but free books.
As for me, I've been reading most of the FREE books online with the MobiPocket Reader. I happened to see a deal for a Sony Pocket Reader (PRS-300) and purchased it. I love it because I use it with Calibre and don't have anything thing to do with the Sony Reader Software. I have loaded over 100 books into the 300 and am having a ball with it.
So if your main interest is reading Free books, I'd say buy a Sony Reader but if you strictly want to be able to buy new books and have them sent to your device, then buy a Kindle. Knock yourself out! :-)
Calibre Link =
http://calibre-ebook.com/