I do use SpamSieve, with an always-running Mac mini (in the last few years with the remote training scripts, so that I can train it from my iPhone or iPad when I am away from home.) But I use it only on the Mail accounts that already have poor spam server-side spam filtering - the ISP account that my wife and I share, a old Yahoo account, and some accounts on my own domain that don’t filter spam well. For me, it’s been a fantastic product. Our ISP email account in particular gets so much junk that it makes using it on a mobile device almost painful without spam filtering; I was using a windows mobile “smartphone” at the time, so it’s been useful to me for that purpose basically from the start. I think I have to move a message to my trainspam folder maybe once or twice a month, and it’s far more rare to have a message be marked spam that shouldn’t be.
I don’t really need it nor use it for my Gmail account or for my Fastmail account - both have great server-side spam filtering that seems to learn from me marking the occasional improperly marked message as good. And I do have an iCloud account that I don’t use often, but, again, haven’t started using spamsieve with that account.
I just upgraded to version 3 and when I was looking for my license info I found that I’d purchased my version 2 license in April 2007 - so I’ve now paid $50 for 16 years of spam filtering. For me it’s easily one of the best valued Mac apps I’ve ever purchased.