I've been thinking about getting the Sony eBook - sat next to a guy with one on a flight late last year. One bonus for me is that I'm avid S/F fan, as was my seatmate. Turns out that Baen makes a number of electronic titles available at no charge. Not sure if you can load them on Kindle or not...

http://www.baen.com/library/defaultTitles.htm

Cheers,
Jeff