i think better to use captcha too to filter the spamming . i had use wp-recaptcha for spam filtration and now it has worked. Before that askismet puts spam in spam category but now no more spams.
Stopped now but that day I was getting ALOT of spam. Anything which makes it difficult to leave a comment can't be very good and I actually find Akismet to work well the majority of the time.
Akismet works great for me.Sometimes it puts comments that are not spam into the spam folder but not often.