Akismet became useless for me after I had two websites get to be rather large (as in, 8+ years of data).

I had been using Conditional Captcha to cut off Akismet's comment histories, which had bloated my databases to ridiculous levels, but I got tired of always having to optimize my databases so frequently because Akismet couldn't clean up after itself.

I don't even install Akismet anymore, and since then, I've noticed a TON of 404 errors from probing attacks aimed at Akismet files, and I'd love to know why that is.

I've used either WP-SpamShield or ZeroSpam, depending on site needs for the past year or two, and I'm delighted with their performance. I've begun looking at newer options for protecting login attempts, and all the other goodies.

Observing the logs for the 404 errors from people scanning for specific files in specific plugins that have previously been exploited is a fascinating read... I had no idea that so many free plugins were targets.