I admit it. I've never used Flex. I don't even have Eclipse installed and I've never really used it except for some J2me experiments a few years ago. It's never been installed on my current laptop.
For the past 4 years I've written a pretty major web application in Flash. It's really all ActionScript - 400K compiled code of it. I really only use Flash as a resource holder. I occasionally do use real Flash to make some objects a little more, well, flashy, but that's rare. Anyway, the web app has done fantastically well. It "owns" our vertical market for what we do.
So I'm about to start on the next generation of this. The next version is really more about being an Air app across all places where Air can live. I've been getting up to speed on the latest CS5 stuff, watching videos, exploring, and I'm feeling like I'm in the wrong world. So often, people are segmented into Flash=designer, Flex=developer.
I am definitely a developer. 30+ years of it. So before I begin on this new, large Air development, am I dumb to stay in Flash? What am I missing in the Flex world? Is it worth the pain to get all used to the tools/Eclipse/FlashBuilder? I've grown very accustomed to external file ActionScript development (thank you TextMate) and don't feel like I really need the integrated thing that Eclipse provides. The only thing I don't like about Flash CS4 and previous was the debugging. I tend to do a majority of debugging by trace statements. Seriously. I'm really hoping CS5 is better with the debugger (getting it this week).
So any advice?
Your easiest route to AIR will be to stick with Flash, or you could use FlashBuilder with a pure AS3 project, or use FlashBuilder within the Flash IDE.
I'd only advise you to use Flex if you decide to re-engineer the whole project and coming from a non-Flex background that will take some time.
I would stick with Flash and not introduce the Flex framework for your first AIR project.
Flex seems to be a good fit for us non-developers. But the more I develop, the more ActionScript I employ, finding the mxml components a bit klunky, although convenient and full of built-ins. I guess in the end, it all ends up ActionScript anyway.