I spent a few hours trying OpenFL + Flashdevelop (which I already use for my AS3 AIR projects), and made an attempt at converting some AS3 code to Haxe (with Flash as a target).
I can't say things felt very smooth. 🙂 Haxe is certainly a powerful solution, but there are so many options that it can become confusing pretty quickly.
Basically, I tried to convert a class from AS3 to Haxe using AS3HX. First I got some weird crashes with AS3HX (I searched for info and saw that some default parameters must be avoided in the command line). Once it worked, it helped on many things, but it also introduced many unwanted instructions that were supposed to enhance compatibility, but were actually not necessary and even caused issues. Also, there were some formatting bugs that deleted some "{" which broke the whole code. From what I saw, don't expect a perfect automatic translation without adapting your code.
However, I was surprised to see that my SWF file generated with Haxe ran fairly faithfully after this adaptation, which is nice! I wanted to go a bit further, and tried to configure Flashdevelop, so it considers it as an AIR project, and launches the ADL tool when I press the F5 key. But despite trying different settings in the Project Settings window, none of them worked as expected. I even tried to launch my project via command line with "openfl test air", and got a cryptic "TypeError: Error #2007" (my SWF file runs fine if I do "openfl test flash").
Finally I tried to use an Haxe extern to use the greensock library, and even if I managed to compile it properly, I now get a final "TweenLite is undefined" error when I try to run it (I asked on the OpenFL forums for help).
Of course, all the issues above can be mainly caused by my inexperience with Haxe, and your mileage may vary. But I spent hours looking for solutions myself, and that's where I realize the information is scattered everywhere, and that you have to do some kind of clues hunting here and there. Beginners might get confused or discouraged. Moreover, I got some errors with some tools (like AS3HX) which were definitely bugs. The fact the "openfl test air" command doesn't work too is pretty puzzling, as I defined all paths properly. So sometimes it felt like the paint was still wet and that the solution was still a bit a work in progress.
Sounds promising though, but like with every tech, you see that nothing is perfect when you try to concretely use it. 🙂
(no offense intended, just sharing my personal experience, for what it's worth)