Scribd was designed to share documents, and its basic assumption is that you're going to read them on a PC of some sort. It formats the documents in a preview window that's pretty and readable— it looks just like a print book— but it's not terribly easy to read a window-within-a-window. I had to line it up carefully so that the window fit my browser screen; in some books, the preview window wasn't big enough for the book's page size. I could click the button to turn the pages, but I had to use the scroll bar to be able to read to the bottom of a page. If there was a way to size that window, it wasn't obvious.
Second, the Simon & Schuster section of the Scribd store didn't have any good way to search it— no "by genre" buttons, not a lot of sort functions, and when you search, it didn't look like you were searching only S&S books. Also, the books are way more expensive than on the Kindle store. I saw lots of books over $20, including a nonfiction one on John Brown that was over $40.
And what do you get for that? You get a DRM-protected book you can read on your PC, that's what. It says it supports EPUb and XHMTML and Adobe editions, but no mention of Mobi, so I didn't buy anything.
So, my analysis of Scribd is that it's a great place for free books but not that good a bookstore. Still a lot of work to do in that department.
