I think an understanding of the others options out there is really good. I saw the title of this thread and before clicking it was hoping it didn't turn into one giant flamewar, which of course, it did. I think it is worth everyone who is involved in Apache Flex staying up to date on what the HTML/JS alternatives are. I haven't yet played with Sencha, but I certainly intend to.

Like someone mentioned, Ted Patrick (formerly Flex evangelist) is there and is reaching out to current Flex devs to provide support for taking your existing Flex knowledge and transitioning that to Sencha. I'd argue that every committer on this project should give Sencha a shot (and all the other realistic HTML RIA frameworks). If you reached out to Ted and said you were a Flex committer I bet he'd be willing to throw some licenses for the Sencha tools your way and help you learn the basics (obviously he wants to convert people, but you can still take advantage of that to learn and apply what you learn back to making a better Flex framework).

Don't discount HTML/JS frameworks without investigating them, or you'll find yourselves contributing to a waning project while the competitors are recruiting all your developers.