After struggling with third-party solutions and two native UIs, I decided to give Hot Door's CORE a serious effort. The fact that the entire code base is cross-platform and supports many different versions of Illustrator at once, and even includes a UI API that makes a great replacement for ADM. The code seems a bit verbose with having to drill through so many levels of subclasses for most functions and declarations, but I don't miss having every other line read "if (error) goto error." There are some really useful functions not included in the Adobe SDK, but although a few items are not yet there, it's possible to use SDK suites in addition to CORE.To be honest, rewriting so much code up front is a daunting task, but I expect things will be easier and better in the long run.
On the downside, Hot Door will update CORE for their own products as soon as bugs are found or things change in a new release of Illustrator, but CORE will be available to the rest of us in increments, so far 12 versions in the past two years. Once CORE is updated, however, it will take minimal effort to update all of our plugin variations for a new version of the OS or of AI. The real advantage I see to CORE is the abstraction of differences between platforms, versions, bugs, the much-missed ADM, but this overhead adds about 12MB or more to a plugin that would probably otherwise be a few hundred K. Then again, that's not very significant on a multi-terabyte drive, and roughly equivalent to one high-res photo.
Supporting so many versions of AI requires more versions of OS X targets and compilers, available with the help of XcodeLegacy script which works very well with even Xcode 7 for compiling for versions back to Xcode 3. Windows requires Visual Studio 2010, which I've found difficult to find for less than four times what I paid for VS 2013 Pro. Maybe you'll have better luck than I did at adding the V100 toolset to 2013 or adding 64-bit support to VS 2010 Express. (I'd greatly appreciate if anybody could share any tips in this regard!)
Developing AI plugins is much more difficult (for me, at least) than it used to be, but thanks to CORE, I'm more productive than I've been in years--seriously. I highly recommend giving it consideration if you're unhappy with your current tools.
ADM has been replaced now with HTML5 panels. Those should be pretty cross-platform.
Thanks for the response, btempleton. I'm sure that's the route I'd go if I were just starting out now. ADM was a pretty broad set of suites, covering much more than windows and widgets, and I need to update my plugins from CS6 on, which complicates things. CORE addresses all of those UI and version issues and has other pluses as well, so I have little interest in leaving it. Still, knowing the underlying SDK helps me get more out of CORE.