Short answer, Yes. There are already many prime-time Apps using Flex as their UI development platform. If you go to the Adobe site they showcase quite a few.
Speaking personally, I chose Flex for two reasons, first was that, although you probably can do much of what Flex does in HTML or with an appropriate toolkit, Flex is designed for attractive and compelling user experience and has available all of Flash. Plus the development environment and available widgets make it easy and fun to program. I don't want to spark a religious war about HTML vs. Flex, so I'll leave that there - it works for me and my application and customers.
Second, and more important, was that it balances the processing load more towards the client which means my server architecture can be optimised just for serving the content and persisting the data. Most of my business logic has migrated across to the client. Having spent many years in classical architectures I think this is a huge step forward, but I can already her a chorus of disagreement about that too.
My word of caution about Flex comes from needing to adopt the right architecture for your client code. It is pretty easy to create a huge and badly performing app with Flex if you get that wrong. Make everything event driven and apparently asynchronous and you should be OK ('apparently' because the Flash player is single threaded). And that is downside 1, the single threaded Flash player sometimes causes issues.
Downside 2 is perhaps more serious and that is locked down desktops in corporate environments. Quite often your target audience won't have administrative rights to their computer and will have either the wrong flash player or none at all. This is particularly true in public sector organisations and the military, so if you are heading there I would test carefully the presence of Flash amongst your users.
Other than that I heartily recommend Flex. It's also a great thing to have on your CV!
HTH