JMS is a very robust and useful extension for Joomla. It is a must for any serious developer who wants to design, host on the same provider, and maybe even resell many websites. JMS gives you great flexibility in the ways you can setup the master-slaves websites relationship. You really have to study the manual (very good and detailed) and the tutorials (including videos, high quality) first to get the full advantage of the extension. I want to specially mention outstanding support of the developer. His response is very quick, he's polite and professional, and he really tries to answer all the questions (no matter how dumb they are :). He spends time with you until the matter is resolved.
It is obvious that the developer is taking good care of his product and his customers. Highly recommend!