Id be surprised if akismet let's pass more than 1 for each 1000 spam comments.
If that happens than you know you found a spammer who knows what he's doing, which isn't the case 99.9999999999% of times.
That's why I love wordpress and all their documentation instead of custom code with no docs.
+1 (I normally don't do the + thing, but this is the second one in the row for me. I did them cause I agree with the post(s))
Akismet does work really well. I have used Wordpress for some projects and it is a neat piece of software. Check also the add-ons, you have anti spam tools in there too (like captchas, etc).
Truth is, regardless of the software you run, you will have to put up with spammers.
My 2btc