Akismet has worked extremely well for me over the last two years, it’s only inaccurately identified two comments as spam out of almost 20,000 comments. Akismet is pretty much the only plugin used on every WordPress blog, and works for almost everyone although it is no longer free for business use.
There’s been reports over the last year or so of comments missing for some people, but I haven’t ever experienced this or heard from anyone I know of comments disappearing.
Although, extremely good Akismet won’t always solve all spam problems especially on sites with a lot of traffic. I usually recommend using a capatcha with Akismet.
What do you recommend or use?