The mobile story in Flex is quite strong, in my opinion. It is what has attracted me to the platform and what seems to be bringing life into the Flex community.
The experience for developing apps in Android and iOS is quite fantastic, actually. The velocity in which you can dev is blazing and the abstractions provided by Flex (data binding, state management, skinning, etc) give you the ability to totally rock your app.
The performance is better than I had expected. It is not as great as a native app, but it certainly gets the job done. The ability to share code and UIs between Android and iOS more than make up for it in most cases.
There are, however, limitations. For one, you are not using the native widget set. You are using the Flex widget set. This means that you do not get the native look/feel. For this reason, it is best to build apps that look like YOUR app... not a Flex app or a native app. There are lots of popular examples in the app stores that work this way... and a Flex app pretty much requires it in my opinion.
There are also a lot of APIs not available to you. Flex provides hardware abstractions for the most popular APIs (video, audio, accelerometer, positioning, webkit, etc) but platform specific APIs are still missing (contacts, calendar, system notifications, etc).
At that point, it is worth asking what your app needs to do. Does it have a lot of native interfacing? 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-centric display/edit app, then Flex is a strong fit.
Hope this helps :)