I have downloaded Flash Builder 4.6 and CS Extension Builder 1.5 and we have one big problem related to design mode.
Actually when we switch to design the visual editor is completely black!
It's not going to be fixed.
The only version of FB where design mode really works is 4.0. Even in 4.5, it's very fragile for Flex 3. (Flex 4 should work fine.)
Yes. I use EB 1.5 all the time. The lack of a working Design Mode is annoying and makes devlopment somewhat less efficient, but it's not a show-stopper...