I just read an interview by Michael Desmond of The Redmond Developer  with Brad Becker, Microsoft's director of Rich Client Paltforms (Silverlight team). Interestingly, Brad came to Microsoft from Adobe (Macromedia Flex team actually).

Brad spent many years building client solutions with Flex and he says this - "But what I was running into was Flex was really good for starting a project, but it was really hard to finish anything with it. You'd start running into issues with performance and with scalability and things like that. So we'd end up having to go back to the metal. You'd have to dig into Flash itself and hand-tweak things iFlash, and then you'd be back into the morass of movie clips and timelines and cell-based animation."

So was there frustration using Flex? Brad said, "Flash was designed for doing cartoons on the Web; It's actually really good at that. But at the end of the day, anytime you use a high-level framework, there's always times when you have to go below the framework back to whatever is underneath. So it was still a pain."

When it comes to enterprise-grade RIA for business critical functions, Flex and AIR have ways to go. Even Silverlight, whose first target has been video rendering (e.g. Beijing Olympics), is yet to prove as a industrial-strength platform for mission-critical RIA for large enterprises.

Curl, on the other hand, has been deployed successfully at over 400 large global customers for such high-performing, secure, and scalable applications.