I'm not really looking for freeware alternatives, as I'd rather have experience with the professional tools, as that would be more useful for me in future jobs.
Flash Builder is the main official programming environment for flash, and is based on flex. Good for 'professional' experience (good for debugging). Also for professional experience, learning how to cope with Flash CS5 is also pretty useful. Others can probably give you more educated opinions.
In spite of your disinterest in freeware products, which I think is very unfortunate, and your desire that we recommend products on the basis of their commercial use, which I find distasteful, I will mention
flash develop, which is uses the Flex SDK, and which is open-source, and nonetheless quite widely used by both commercial and non-commercial developers. (Interface-wise I find it superior to Flash Builder, though the latter has somewhat stronger debugging capabilities).
If you want to know more about the difference between flex/flash builder/flash professional/flash develop, I'd recommend some googling as a first port of call.