You can do a lot of the same creative material in Flex that you can do with Flash. In some cases, especially if you can handle coding easily, it's even easier, because you can add effects without having to tweak a timeline. You can use the graphics class to draw the kind of simple shapes Flash lets you work with and you can style them.
You can also add any bitmap or vector images you want to Flex and use them however you want, even as background-images for, say, a Canvas. I create graphics symbols in Illustrator and export them to SWF files which can then be embedded and accessed directly in Flex using the [Embed] directive.
It involves a different way of thinking about it, but let me put it this way: I started off as a graphic designer who could code, and loved and used Flash to create cool interactive animations that used complicated and extensive ActionScript. Then I started working in Flex and once I got to a certain level of proficiency I completely abandoned Flash. I don't use it at all anymore unless I have to work with someone else's Flash files directly.
If in your HTML/Javascript/CSS work you're the kind of person who codes in a text editor first, Flex is going to feel right for you. If on the other hand you rely on DreamWeaver or some other visual tool, you may want to stick with Flash. Bear in mind that Flex also has a fairly rudimentary "design" view, which I never use because it just gets in the way. As would DreamWeaver.
Edited to say: I should add that you can also create animations in Flash and use them in Flex. So you don't have to abandon Flash to use Flex, but it just might fall out that you do, as I did.
