It's been a hell of a month for Akismet on our site; for some reason we've had more false positives than real spam stuck in the filter, and more spam in the comments than real replies.. So we've decided to give Spam Karma 2 a chance.
I've noticed it has a helluva lot more options than Akismet, and at the very least the logic remains on our server and does not await a reply from a heavily-hit server on Automattic's end. We'll see if this does the trick, but we have high hopes!