I have used ZAGG screen protectors on many of my devices, and while they are terrific, I do think they are EXTREMELY overpriced. I have latey started using BestSkinsEver protectors, and they are EXACTLY the same thing, except much cheaper ... and have a mych smaller selection (but they do add new models all the time)
http://www.bestskinsever.com
However, my biggest concern with putting these screen protectors on the Kindle (or any other eInk) screen is that they will take away the matte "paperlike" finish and will become more glary.