We had many more headaches with real time black lists that definitely outweighed the good of using them.
There were many false positives and it did delay the delivery of messaging.
I found that most real time black lists time out the majority of the time leaving your mail server checking them at least 3 times for each message, thus causing a delay.
If you have the ability to use a weighted system I would leave out black lists and focus on Phrases, URL black lists, content filtering, reverse dns, mail froms, verify'ng ehlo/domain, and statistical filtering.