There’s probably already more than enough commentary out there about Flex MVC frameworks.  Instead of adding my limited knowledge to the noise, I’ll link to other discussions.

http://www.anandvardhan.com/2008/11/13/popular-flex-frameworks/

a good primer discussion of several different frameworks and their basic design decisions.  Includes PureMVC, Cairngorm, Mate, Penne, Swiz, easyMVC, Guasax, Model-Glue Flex, & Ruboss.

http://www.adobe.com/devnet/flex/articles/flex_framework.html

compares Cairngorm, Mate, PureMVC, and Swiz with a well-thought-out opinion of strengths and weaknesses of each.  Somewhat surprising coming from an Adobe site, but they’re not trying to sell Cairngorm.

http://www.asserttrue.com/articles/2007/10/17/silvafug-application-frameworks-presentation

A great presentation by Ali Mills and Luke Bayes of PatternPark which helps narrow down the list, and has a discussion about PureMVC and Cairngorm. Summary: they really like PureMVC and dislike Cairngorm.  I think there’s pretty much a consensus that Cairngorm is the EJB 1.0 of Flex framworks.

Frameworks they eliminated due to lack of documentation or community:

• Slide – doesn’t exist
• ARP – by Aral Balkan, abandoned
• Servebox Foundry – not really an MVC framework (they later say it’s worth a second look)
• Guasax – too early stage, runtime XML configuration (a la Spring), only in Spanish
• Flest
• MVCS – by Joe Berkovitz – a blueprint not really a framework – definitely worth reading the article though
• Model:Glue – port of CF framework, not MVC

Their summary also says they don’t use any particular framework, but that understanding the patterns is important, and Flex itself is often good enough without another framework if you can follow the proper patterns and make correct architecture choices on your own.

If you only look at one framework, look at Cairngorm (because everyone else is using it), but if they had to pick one framework, they’d choose PureMVC.

Note: my characterization of their comments does not necessarily accurately represent their opinions.

http://robsondesign.com/blog/index.php/2009/04/02/puremvc-vs-cairngorm-revisited/

Like the title says, a comparison of PureMVC and Cairngorm, which seem to be the two dominant frameworks.  Mate and Penne seem popular as well among the fans of simplicity in a framework.

http://www.adobe.com/devnet/flex/articles/blueprint.html

Joe Berkovitz’s discussion of MVCS mentioned by Luke & Ali.  It’s definitely worth reading, and includes a sample app that shows a concrete example of his ideas, if not an actual framework.

http://corlan.org/flex-frameworks/

A good list of frameworks, not just MVC, but everything from testing and continuous integration to  remoting  and 3d engines.

http://myflex.org/presentations/ComparingFlexFrameworks.pdf

A comparison of Cairngorm, Mate, PureMVC, and Clear Toolkit, by (I think) the creator of Clear Toolkit.  The first three are MVC frameworks, but Clear Toolkit is a library.  Has a very good architectural overview of each framework.