I write a lot of informational articles and reviews. The most rewarding of the informational articles are the ones I've written on tropical fish, they're one of my favorite subjects because there's so much misinformation out there. When people comment to tell me how much I've helped with their fish it makes me feel great, and like I've just eliminated one more chronic fish-killer from the world.
For reviews, my most rewarding moments were from three of my book reviews -- the three that garnered positive comments and thanks from their authors :). In one instance, the author even told me that she was glad I liked the book, but that it was her first effort and she's published much better books since then -- and sent me a copy of her newest, no strings attached :D.