There's a lot to do to SA to make it "good". I shared your opinion a year ago. I run a relatively low volume personal mail server for a few domains and a few users. I had SA, but it didn't do much, and I had bigger fish to fry dealing with much larger mail sites than my stupid personal nonsense. I typically get about 300-500 spams a day, and very few legit mails. I was getting false positives, so I'd just never see the mail, and tons of false negatives. About 20% of the daily spam was hitting my inbox, making it unlikely that I'd ever even check my personal mail. If you mailed me, and I didn't have an existing filter from you, there was maybe a 60% chance I would notice your mail in time for it to matter.
I decided one day to fix all this, regardless of what that entailed. I lowered the threshold for SA to a score of 4 (which they bark at you not to do, but fuck 'em, I've seen maybe 6 legit mails with a score higher than 4.5, in my world anyway). The key components were: enabling remote checks, RAZOR and DCC, and having SA train its filters off of my false negatives. I use the Train SA
script, so I drop any false negatives in a Train Spam folder, and this picks them up and runs them through SA's filters to train it.
My false negative rate dropped pretty much immediately from 20% to ~3% to 5% on weekdays, and zero to 1% on weekends, which I can live with. In the year or so since I actually put my back into fixing this, I've gotten maybe 2 false positives.
I don't see long processing times, mail comes through pretty much as I send it in my tests on my VPS, but again, I only get a few hundred mails/day. If I had volume over a few dozen thousand/day, I'd probably just bite the bullet and pay Google (Postini) to make it go away.