<p>I came across the Adobe Flex framework one day.  After watching http://www.adobe.com/products/flex/media/flexapp/'> the demo of Flex,  it looks to me it is a bundle of different Ajax controls.  I have no working knowledge of Flex at all and know very little about Flex.  Therefore, I would like to hear from the developers here with some Flex experience to explain a bit more pros and cons of this Framework.  In particular:</p>

<ul>
<li> How productive it is to program in Flex compared to the .Net + Silverlight? </li>
<li> Any technical advantages over other frameworks? </li>
<li> Any disadvantages ?
<li> Does it have any known scaling issues? </li>
<li> What kind of web servers can it be hosted on? </li>
<li> Any other things I should be aware of about Flex?
</ul>

Well ~~ .. it's more that Silverlight is similar to Flex. It's not Ajax so much as a kind of 'walled-garden' inside of which you have a rich programming environment. Somewhat like Java apps say.
The usual criticism derives from just that. Flex/Flash applications tend to be ignorant of the browser (more or less) so that stuff like back-button usually gives unexpected results.
It'll run on all the current browsers on all platforms. You don't
need
to "host" flex as you build a SWF file that gets loaded into the browser and is executed by the Flash Player which your users are expected to have already downloaded/installed .. (95+%) penetration helps.
I've been using Flex for the last year or so and it's served my purposes well. It can be integrated nicely with javascript stuff on your web pages and provides a very rich set of capabilities for doing all sorta things like web-services and XML processing and of course all your video/audio/music rich media.