Look for a longer article from me in the near future on Spamhaus; I'm collecting a ton of data against a large spam corpus and hope to summarize and publish my findings within the next month or so.
Until then, feel free to bop on over to Spam Resource, where I talk about my experience using the Spamhaus ZEN blacklist to tag and filter inbound mail to our abuse desk. I've been quite pleased with the results.
Also of note is that Microsoft is using both Spamcop and Spamhaus to reject mail to their corporate users. (They're NOT using it on MSN Hotmail.)
Update: Find my full review of Spamhaus ZEN here on DNSBL Resource.