Funny, since I upgraded this blog and moved to use Ajax for comments, I don't need a captcha.
I noticed that still my blog gets a number of spam-attacks per day. But, it gets refused because the viewstate doesn't match. So, for now I don't do anything about it, because mainly what I see is automated post-attempts that post directly to a page without viewing it first. Therefor, they have no viewstate, thus the post gets rejected.