As a fastmail user I was waiting for them, but now knowing its either labels OR folders, I will stick with folders. Labels tend to be much more disorganized for me. I consider them an extra on top, not essential
I kind of use them like folders, but one main difference is that I can keep important emails in my "Inbox", while they can still show the label. With folders, they basically become archived right away, so there were emails that I was hesitant to assign a folder to, because it'd mean that they'd disappear from my view.
Basically the "Inbox" is my TODO list in a sense. And with labels I can keep better track of what is necessary for the emails/todos to become archived.
