I have been using Acrobat as my primary pdf reader because of the markup tools it provides and the ability to create "real" bookmarks. After exploring Skim, I am switching to it as my default reader. It is certainly cheaper, but more importantly, I think it's features make it a better reader. It doesn't create "real" bookmarks, but it's notes feature is almost equivalent and is more convenient. The snapshot feature is a big aid to anybody reading scientific literature, but helpful to others also. Little features like the magnifier tool are also nice.