Great place to get started @ http://blogs.adobe.com/flexdoc/2009/06/documentation_for_the_flex_4_a.html
Experience so far: Installed it and compiled 3.1 projects…a bit slower than usual…maybe a lot slower. Disappointed that the network monitor stuff only works for new 4.0 projects. There is a lot of developer features to take in. I haven’t been this excited since Flex 2 Beta 2.