I use TextMate. It's not free, but it is affordable, and it works well.
My experience with SEPY for mac was that it had too many bugs to program all day in. For casual edits it's fine, but anything more than that ...
I suspect flex builder is the best environment for the moment, but it's a bit too pricy for my wallet (I only do hobby projects on my mac, nothing commercial).
Would love to use flexbuilder, but can't afford that and flash. Flash Develop looks great but not available on the mac. I switched about a year ago and this is the first time I've wished for a PC, and that's just not right.
If you've got an intel mac, you could always get parallells and run flash develop. It's not ideal (and personally I'd rather stick with textmate), but it's an option.