I like RDNSBLs - they are extremely useful, and when used properly, they can reduce your SPAM intake by 90% or more easily. When they don't work well though, they kind of suck. No, actually, they really suck. One big problem with SORBS is it's overly aggressively blacklist of supposed dynamic IP addresses - many of which are not dynamic. Add to that the mandatory registration process required to de-list an ip, the molasses-like slowness of their website and their moronic use of a self-signed SSL certificate. I would consider it a joke if there was humour to be found in the situation.
While the use of SORBS might offer some SPAM reduction - I do not think it is worth the additional hassle, there are plenty of other perfectly good blacklists out there to choose from - SpamCop, SpamHaus, UCEProtect being a few.
SORBS was also acquired in 2009 by GFI Software.