I was using the trial of SpamSieve for some time; I can say that it picked out more spam than Apple Mail did. But not significantly more. So, huh …
I think using a tool like SpamSieve is simply the wrong approach. It leads you to false behavior, concerning your mailboxes and your mail behavior in general. The only valuable approach (in my book) is to maintain a certain discipline: When subscribing to anything that is just one-off or suspicious just don’t use “your” mail address, instead use any of the available throw-away addresses like @mailinator, @googlemail, @gmail, @yahoo or something alike. Subscription to these services is free, you can open as many spam boxes as you like/need. This way all the spam will land in the gmail or yahoo mailbox and won’t bother you any more. So simple.
When your mail boxes are already flooded with spam, it’s (probably) too late. Tools like SpamSieve may help you temporarily, but, of course, they won’t help you at all to get rid of the BS.