Mike highlights the big problem with mailblocks. You see they have NO SPAM filter. They simply challenge every piece of mail you get, and whomever can figure out how to prove they are human gets to have their message delivered to your inbox.
Well the problem for me is the sheer number of spams I get and the number of e-mails that are not sent by humans that I care about. I have taken the unusual approach of having all my mail forwarded to fastmail.fm, my main e-mail host, have them scan it using their spam filter (spam assassin), which gets rid of 80% of my spam, then forwarding anything else to mailblocks which gets challenged.
Well the problem is that I still have to scan my pending folder all the time to pick out mails that I care about. There are enough of these that I just don't get the value of challenge/response. Additionally, about 6 spammers a week actually go through the challenge response and I am still getting spam from them. That just sucks.
So, I have decided to discontinue my use of mailblocks and switch most everything over to my hotmail account. The reason being, hotmail has the best server side Junk Mail Filter on the market... and I am not kidding here. I get an average 5 spam messages a week in my hotmail account, which is much easier to deal with than the 30 items I day in my mailblocks pending folder (the 20% of the spam messages that fastmail's spam assassin filter does not get). I estimate that hotmail, when set to Enhanced gets about 93% of my junk with very few or zero false positives. For me, I get on average of 180 spam messages a day! So, every percent matters.