I have noticed it for sure. The default spam filter catches most of what I'd consider spam, but the Learn Spam feature is needed for things that get through because they look legitimate. For example, I get a lot of those weird "You're an American, I'm from [China, India, etc.], we can make a lot of money if you go to all the interviews/meetings and then let me do all the work" kind of emails. They look like normal correspondence (maybe they are) so they occasionally end up in my inbox. When they do, I send them to the "Learn Spam" folder, and the next time I check my actual Spam folder I'll find that Fastmail caught several more just like it and sent them straight there.