The Kindle reads large, image heavy A4 PDFs very fast (just like my MBP w/ SSD). I'm impressed with the performance. To reading experience is not that good. Switching to landscape, zooming and scrolling - it's a hassle. Good for looking up stuff, but not for reading.
Tested books: "Pathfinder Core Rulebook" and "1601-1700, Standard Catalog of World Coins, 4th Edition"