a new feature of flex 2 is “view” states whereby i can layout multiple views on the same component using mxml (or actionscript) and change state by setting the currentState property on my view. you can also specify which transitions to use etc when switching from one state to another.
viewstacks in flex 1.5 (and 2) sortoff offer the same functionality.

in flex 1.5 some of the people who work with me tried to create 40 switcing icons that switched between 2 states.they quickly discovered this was a bad idea since all the movieclips got created at once, then the flex framework kicks in with its layout logic, so suddenly the time it took before the user could see a screen jumped from 1 second to 8 seconds. so the next approach was adjusting the creationPolicy, but this let them down an axis of evil (creationpolicy is not always as  straightforward as expected). so what they did was create the clip in flex and use the embed directive. problem solved.
(this could have been done in flex 2 by creating a mxml component with 2 states.)

this example however highlights a limitation of viewstatcks in the past (i have not looked into the dynamics of viewstacks in flex2 so am not certain if all the views gets created immediatly but i suspect so) and maybe in the present. in contrast to this (view) states add and remove the clips as is needed. this is awsome in my opinion. also the zorn plugin for eclipse allows you to look at states independently when designing a form. its just been pointed out to me as well that the design view in zorn allows you to toggle between viewstacks as well