I think there is a chance they will gain momentum. As long as there are multiple browser vendors with significant market share interpreting/implementing specifications, web development is going to continue to be painful. As evil as a single-vendor proprietary platform sounds (is?)...it sure would make a lot of things easier.

I've started looking into moving an application to flex for two main reasons: #1 to support no javascript. As more and more ajax features creep in, it gets harder to always provide a noscript fall-back. #2 to do away with cross browser compatibility issues.

Some concerns that I have with Flex so far are #1 no html rendering built in...so there is no way to leverage existing static html/css resources without converting to flex ui (I've read that AIR comes with Webkit, but flash in the browser does not.) #2 there doesn't seem to be good support for printing, especially compared to simple print stylesheets. Oh, and the other huge concern is how prohibitively expensive flex developer is.