I've been using Flash for a month or so, and most of the concepts began to sink-in after a few weeks of intensive practice.
I bought a few books on the basics and bookmarked dozens of sites I found in google searches.
Personally, as a designer I now vastly prefer working in Flash compared to WYSIWYG website editors such as GoLive or Dreamweaver.
The main strength of Flash for me is ability to design freely without having to worry so much about the client browser, html formatting etc...
The designs always look the same regardless of the browser or platform they are viewed in.
Actionscript is actually not that difficult to implement once you understand the basics.
The first thing to master is proper browser detection and express installation of the flash plugin.
http://www.adobe.com/devnet/flash/articles/fp8_detection.html
BTW, I tend to disagree with the blanket assertion that html sites are somehow superior to Flash sites in terms of usability or functionality.
In many respects, a preloaded flash site is actually far more responsive and free from errors than many complex html sites, in which links produce a blank page while the content is loaded.
(this is super-lame application behavior IMO, but people have been conditioned to accept this as being a part of the HTML web experience)
There is also the issue of client browser settings, versions, or configurations causing html sites to render inconsistently.
There is almost never an option to easily upgrade/modify the browser to render the site predictably, as there often is with Flash sites.
Take the Mac Rumors Forums site for example:
Stretch the Safari window to completely span the width and height of a 24" iMac.
Now look at the black stroke running along the top edge of the navbar at the top of the site; on my system the line ends prematurely just before the "search" link.
Insignificant, but it illustrates how html sites often fall apart in unexpected ways.
HTML was originally developed to allow scientists/researchers etc... to share data between obscure systems that could not otherwise communicate directly.
Not only is this really not actually the case with most contemporary websites, the idiosyncrasies and limitations of the aging HTML standard has created the perception that web applications (or websites) are the red-haired stepchildren of "true" desktop applications and games.
This may actually be true for most traditional html sites, but Flash sites are significantly closer to the desktop experience, and it's easier to achieve desktop-like responsiveness and advanced GUI effects in Flash than it would be in AJAX.
The fact that Flash sites are often too much like riding a rollercoaster is not an inherent flaw in the format.
I think it's just too easy to do, so lots of designers do it.
BTW, the Leopard GUI vs MacOS9 GUI is in many ways analogous to the difference between Flash and traditional HTML websites.
A "paradigm shift" causes what we hold sacred today to be viewed as quaint and obsolete tomorrow.