I was gettting drowned in spam comments...weathervane, safety signs, SEO stuff, custom t-shirts, etc. I added Akismet and, tada! All of those annoying comments are filtered into my spam folder. I still check through them regularly to make sure nothing "real" has landed in there but so far, so good. It's really worked.
I ended up paying a small amount to Akisment (payment to them is all up to you...nothing to a few dollars) because I just feel that it's been an extremely helpful plugin and they deserve something!