I'd tried Akismet before, and wound up reverting to Spam Karma 2 - actually, I think SK2 was interfering with Akismet, so that likely wasn't a fair comparison, since both were running at the same time.
But, I've been running the Drupal Akismet module for 10 days now, and it's been performing absolutely perfectly. For example, this blog has been under a sustained spam attack for the last 12 hours or so - over 400 600 spam attempts just last night (200 just while writing this post) - and not a single one of the roaches got through. I just went through the Akismet comment moderation queue to look for false positives, and there wasn't a single one. So it's batting 1000 under a significant spam attack.
Mad kudos to Akismet, and to Markus for porting it to Drupal!
Actually, I'm pretty impressed at how this blog is running under this spam attack - it's still responsive, pages load quickly, and posting new content and comments is still working. Drupal's handling the extra load without breaking a sweat.
I'lll be keeping the Akismet module running on my blog, but will keep playing with the Spam.module update on our campus server due to licensing requirements for Akismet. Our use falls under the "commercial" category, and with the number of Drupal sites we're using, the cost can't be justified (yet - maybe if we get hit by spammers that don't get blocked by spam.module we'll adjust things to find the cash).
Update: Over 2000 attempted spam comments in the last 24 hours, and every single one was stopped by Akismet. Here's a screenshot from the Akismet moderation log - I didn't have to see any of these, and they were coming at me every in bursts of up to 1 spam per second for 24 hours...
Akismet Spam Attack: A screenshot of the Akismet Drupal module moderation log, during a sustained spam attack where Akismet blocked 100% of attempted spams, with no false positives.
Update 2: It's been almost exactly 24 hours since I first wrote this entry. Since then, an additional 2000 comment spam attempts have been successfullly blocked by Akismet. And a whopping 8 spam comments got through to my blog. 8. That's it. Out of over 3000 attempts in a day and a half. That's roughly a 0.267% success rate for the spammers. But, the economics of it make even THAT a worthwhile use of their time.
I debated doing something more proactive to stop the spammers altogether, but then thought that it's probably better for them to leave their bots pointed here, getting no benefit at all, than randomly spraying their spam across the 'net and maybe hitting someone's blog that isn't using an effective spam blocker. The way the Drupal Akismet module works, the spammers think they're getting every comment posted here, but the module immediately unpublishes their spam as soon as Akismet responds. That's a pretty sane way to set up the block - don't tell the spammers that they're wasting their time, just nuke their spam without a whimper...