I do periodically review what Akismet catches. Out of over 40,000 spam comments caught by Akismet on my most popular blog, I have only seen 3 or 4 that were not spam.
On the other hand, spammers are getting more creative and many comments that almost look legitimate are indeed spam that is not being caught by Akismet. I always click on the Akismet spam link to flag those as spam. Any comment that simply says that my blog is wonderful or compliments the theme AND uses a GMail e-mail address is almost guaranteed to be spam.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin