Over the last 2 weeks, I've had to remove over 100 spam comments on this weblog. Over 99% of them were apparently from bot software that spiders the blog to bump up the Google ranking of the target site.
Anyway, long story short, I've installed this plugin (relatively painless), and it doesn't require any modification to the MT code. Just drop 3 files into place, make sure you've got Storable.pm on your system, and configure away...
I'm crossing my fingers on this. I was |-- THIS CLOSE --| to disabling all comments on this weblog because I've wasted too much time removing the unwelcomed spam (and much thanks to Yahoo! for doing SFA to stop their users from spreading this garbage.)
UPDATE: It's been a week, and no comment spam! YAY!