One of the nicest features in FlexBuilder for AS3 development is the error reporting that happens whenever you save a class. It checks for any obvious errors in your code and reports them as line items, complete with a description of what's wrong and where. This is a fantastic tool. The Flash IDE compiler is terrible and misses all kinds of obvious errors that won't show up until run-time. I'd rather see Adobe put effort towards getting a decent compiler in Flash than improving its Actionscript editor.

As devoted fans of FlashDevelop know first-hand, FlashDevelop's auto-completion is hands down the best in the industry. If you haven't used it yet, you should give it a trial run. Nothing comes close and the amount of time it saves and the joy it brings is addictive. I can't code Actionscript with any other editor. Die-hard Mac developers have posted instructions on how to run this Windows-only application (due to its reliance on the .NET framework to achieve this level of auto-completion among its other great features) on their OSX machines seamlessly. Those who have used it know it's well worth the effort.

By contrast, FlexBuilder's auto-completion doesn't come close to FlashDevelop's, but its ability to point out the errors throughout your code meant that you had to either pick one or the other or use both in tandem. However, as I have just discovered, this is no longer the case for syntax.

FlashDevelop has the same syntax exact error reporting that FlexBuilder does (minus highlighting exact lines with little x's). It will compile on save and report any syntax errors in the Results panel, telling you the line, the error description, the file and file path, and you can double click on the line item to jump straight to the line and file.

This feature is turned off by default in FlashDevelop. In order to turn it on, you need to press F10, or go to Tools > Program Settings. Then, under the AS3Context plugin, under "Common", the first item is called "Check Syntax On Save". Set this to True and you're good to go.

Short of spending $500-$1000 on FDT for its outstanding lexical parser which points out all your errors on the fly as you code (but doesn't have as good auto-completion as FlashDevelop, though it is better than FlexBuilder), this is as good as it gets for Actionscript developers (for now).

If you use FlashDevelop, please consider donating to the project, as I have done. Phillipe, Mika, and the rest of the team have created an outstanding product which they have given to the community for free and deserve all the praise in the world for their hard work.