We all know that Flash has been modified as a danger due to all it's security issues, but you can't argue as a software developer on how elegant ActionScript 3 is, at least that's my opinion.
The fact that you can get a free IDE to work in Flash, that is compatible with Adobe Flash and Adobe FLEX projects, that it has it's own package manager to download debug tools, compilers, additional packages and more is just awesome.
It's free, it's constantly maintained and updated, has great support, tons of documentation and you can keep working in ActionScript 3, no matter what happened with Flash and with the new HTML5 technologies.
This tool runs over Java so sometimes it can literally eat all your RAM memory.
Also, it doesn't have as many plugins compared to other IDEs for similar technologies like PHPStorm, WebStorm vs Eclipse, but I believe that's because not many developers are still working with ActionScript 3.
Not much. If you are a software developer, you must have used some IDEs before like Eclipse vs PHPStorm or WebStorm, so you will be just fine.
It's very friendly since it has its own package manager so you won't have to mess with the path or complicated installations.
I have done several simulations with ActionScript 3 since the Flash technology is great for visual stuff, plus my knowledge and experience as a software developer and with Flash made it a good choice for this.