I've had the misfortune to work with Flex - I really didn't enjoy it. It was also incredibly difficult to make Flex apps accessible in a meaningful way.

AIR did hold promise, but ultimately suffered from being terribly cumbersome to extend when you wanted to access native features or platform standard libraries (I've created several AIR native extensions over the years).

However, one of the big problems with using Flex or AIR after Adobe have end-of-lifed Flash is that you're going to lose access to the content creation tools. There are some alternatives (FlashDevelop and FDT for coding, I forget the name of the attempt at an open source Flash timeline editor), but nothing is close to being as good as Adobes own tools when you want to.

If you want to leverage knowledge of ActionScript, people may be better off looking at something like Haxe for cross platform gui app dev.

I would not start a new project in Flex/AIR but I am stil bug fixing existing projects. I use Intelli IDEA, I do not need the drag and drop GUI editor I learned how to make good layouts editing the XML and it is much more simpler to layout components using Flex(or other GUI toolkits , I used Qt and WPF ) then using divs and css(thought he new flexbox in css improved the situation), So in Flex(Qt,and others) you have components like listboxes, dropdowns, DataGrids that are very flexible , you want an item to look different you create an item renderer and reimplement the paint method in AS3 or if is simple enough in plain XML(or using the GUI Designer) In HTML5 land you do not have such flexible components as standard so you need to find an open source or buy this compoenents or implement one. You can say it is easy to implement a dropdown(built in one can't show icons and can't be customized), you hack some divs,lis and some css and it is done. But later you find that you did not consider corner cases, your implementation opens outside of the screen if dropdown is on the bottom of the page, the dropdown has no property where you can ask "show max 6 element, probably you did not add all the standard hover effects, and not implemented all the events, My point is that in Flex and other toolkits you can do a good GUI with good layout using the built in stuff and no hacks(I am thinking at css where some obvious thing did not worked for me until I found on SO that I need to set min-width to 0 so the css engine finding this useless rule would take a different path and display the expected thing). Can you share what GUI toolkits you used and what features you loved from each one? Thanks

I tend to use the native UI for each platform I work on. My general opinion of cross platform UI frameworks these days is that they for developer convenience - they don't help the users. However I'm also aware that persuading the budget holder that users are worth spending money on is hard :)

For the drop downs - when I used Flex I had the same issues over window bounds, in that nothing could extend beyond the limits of the flash rendered window. Did they fix that in later versions?

You can extend over window bounds, what I meant is the dropdown should open up or give you a property to allow you this option, also Flex dropdowns had the property you can set how many items are visible at a time. I learned Adobe AIR because my employer liked it better then Qt at that time and he also trusted Adobe products, moving from c++/Qt (and previous C# Winforms and WPF) to AIR was a downgrade in some aspects but after you learn it you can do your job and implement most of the things fast. What do you use for creating native apps for Windows, I do not know what is the prefered thing this days?