I've been using Flash Builder (Gumbo) a fair amount the last three weeks. It's been great for creating pages, widgets, and so forth on the fly - sometimes I find the workflow more efficient than building in Flash - really depends on the type of project involved, how many transitions, etc. On the other hand, I can also recommend the Gaia framework (Flash) while is a neat way to scaffold an entire very quickly.

Should you explore Flex, you may want to stick with Flex 3 because Gumbo is in Beta. I am not doing anything particularly ambitious, but not everything runs as smooth as one might wish... (does it ever?).  Keep in mind that Flex 4 supports both the new Flex 4 components (code named 'Spark') and Flex 3 ('Halo'), but namespaces have to be declared correctly. I've seen some different takes on this - the official word is here:

 http://opensource.adobe.com/wiki/display/flexsdk/Gumbo+Namespaces

I'm using Gumbo to play with new features such as animation classes, the graphics class Fxg as well as
support for Text Layout Framework - those are all areas that
have some issues to be worked out (at least TLF).

Bottom line: have fun! It's all good... mostly.

--Alan