Is there any advantage or disadvantage to using multiple DNSBL's with ones connection filtering? I just switched from SpamHaus to BarracudaCentral but have added in SpamHaus to test for increased effectiveness. So far there is no dip in performance and no increased load on our SMTP server.
Most people that I know who administer e-mail systems use multiple DNSBLs without any trouble. The advantage is that it provides a broader coverage base. Different providers list different things for different reasons that are all equally deserving of being blocked. The downside is that you have more lists to keep track of (who is still active or not, etc.) and it can impose additional DNS lookups on your resolver. Those extra queries could be significant or not depending on your mail volume. On the primary e-mail system that I manage we check three different DNSBLs in addition to a couple of whitelisting services. We've been doing this for years without any significant problems.