I *really* like IntelliJ Idea and 10 has some fantastic Flex/Flash features.
Visual Studio is world class. It is the IDE to beat. Resharper, Isolator, Rhino Mocks, VS itself. There are so many reasons it is an IDE to love. Adobe is working hard to improve Flash Builder, but I honestly don't think it will ever get to VS levels. This is more that it is built on Eclipse and not something I fault Adobe with.