InfoWorld columnist Roger Grimes gripes about Real-Time Block Lists (aka Real-Time Blackhole Lists, DNSBLs or DNS-based Block Lists. The Exchange Server 2007 term for RBLs is "IP Block List Providers").
The feelings towards RBLs are understandable - you won't be a fan of RBLs if your IP addresses are the ones getting listed, and your users breathing down your neck about bounced messages. It's important to monitor RBLs to determine if your IP addresses are listed in any. (Incidentally, Zenprise for Exchange does this automatically several times a day - configurable by the administrator - and alerts you if any of your IP addresses are listed on RBLs. Your external IP addresses are detected automatically based on messages sent to Zenprise's Zmail hosted service).
Once listed, some RBL providers make the process of getting IP addresses delisted a long and painful one. At times, and in spite of your best efforts and assurances by the provider that the IP address(es) will be delisted, they continue to live on in such lists. On the other end of the spectrum are RBL providers that make the delisting process as simple as entering the listed IP address in a form on their web site, without any checks in place. I would shy away from using the latter.
Regardless, if you are on the other side, as a victim of spam rather than a victim of RBLs, good/reputed RBL providers like Spamhaus (which has its detractors as well - read previous post: "Spamhaus, the E360Insight lawsuit and the future of RBLs"), can help in dramatically reducing spam at the mail gateway - by as much as 90+ percent. This further reduces the stress on mechanisms like content filters (like Intelligent Message Filter or IMF in Exchange Server 2003, and the Content Filter agent in Exchange Server 2007).
Whether you use RBLs to block inbound internet mail from black-listed addresses or not, it makes sense to monitor whether your IP addresses are listed or not on a regular basis.
Read Grimes' column titled "Why I hate RBLs" on InfoWorld.com.