The one that everyone wants dead is the flash browser plugin, for a good reason. It's just a pure clusterfuck at this point. If you're targeting the browser, you're better off with HTML5.
AS3.0 itself is still useful, however. It is like javascript except more sane and readable. The structure is quite similar to C# and Java, so you can mentally migrate to those languages without much problems.
Its biggest plus in my book is that when it is used with Adobe AIR you can target multiple platforms in just a single codebase. You can target PC. You can target Android. You can even compile to IOS in Windows without using XCode. It also compiles to binary so it is far faster, and a bit tougher to decompile than your normal swf.
TL;DR: I don't think learning AS3.0 will be a waste of time. However, the flash browser plugin can die in a fire for all I care.