I'm working on a moderately complex Flex project: base application with 6 modules and one shared library. IDEA 9 (92.105) can't run for more than 10 minutes before it reports a fatal error. Usually, it's only a couple of minutes before IDEA encounters a fatal error in either CSS Support or its Flex support, although other areas also cause it problems. Whenever these problems occur, I send the report to JetBrains.
These problems, coupled with the degraded performance of its Flex support, have pushed me and my colleagues back to FlexBuilder for day to day editing. We now only use IDEA when we need to do something complex that FlexBuilder can't handle.
It's a shame, not only because I've used IDEA for almost 10 years, but because FlexBuilder 3 is such a lousy editor. I hope IDEA's Java support is still top notch. I also very much hope its Flex support improves dramatically over the course of this release.