If you're on the fence about paying for a license for IntelliJ IDEA, here is one more reason to justify the cost of the license. Adobe Flex. I've spent just a little time with it and the Flex 4 Beta 2 SDK and I must admit, IntelliJ as a Flex IDE rocks. Much better than Flex Builder 3. You don't get a Design View with IntelliJ, but I never used it in Flex Builder and it seems of dubious value.
I can't comment on how it stacks up against Flash Builder 4, but Flex development in IntelliJ is a joy. I'll post more as a I get a chance to use it some more.