Flex’s mobile story is strong enough in my opinion. This is what attracted me to the platform and seems to bring life to the Flex community.
The experience of developing applications on Android and iOS is actually quite fantastic. The speed at which you can dev flames, and the abstractions provided by Flex (data binding, state management, skinning, etc.) give you the ability to fully download the application.
Performance is better than I expected. This is not as good as the native application, but it certainly does its job. The ability to share code and user interfaces between Android and iOS more than compensates for this in most cases.
However, there are limitations. Firstly, you do not use your own set of widgets. You are using a set of Flex widgets. This means that you will not get the look. For this reason, it's best to create applications that look like YOUR application ... not a Flex application or a native application. There are many popular examples in app stores that work this way ... and the Flex app pretty much requires this, in my opinion.
There are also many APIs that are not available to you. Flex provides hardware abstractions for the most popular APIs (video, audio, accelerometer, positioning, webkit, etc.), but the platform’s APIs are still missing (contacts, calendar, system notifications, etc.).
At this point, it is worth asking what your application needs to do. Does it have a lot of native interface? If so, Flex might not be right for you. Find the APIs you need to talk to and make sure Flex has an abstraction for you. If it is a data-oriented display / editing application, then Flex is strong.
Hope this helps :)