I am using Jms Multi Site for a client who wants to manage several sub-sites from a central admin panel.
They have one main website, and several sub-sites. Each of the sub-sites will contain a smaller subset of the main articles and products, with a few that are unique to that site.
With Jms Multi Site, and some of the excellent add-ons available, we are putting together a system where they will be able to manage all their VM products, articles (and soon even contacts) via the main admin panel for their master site.
The only thing that they will need to manage from separate sub-site admin panels is the unique menu for each site. Everything else will be handled from the main master admin panel.
Of course, this represents only one of the many ways that Jms can be configured for multi-site management. Many people are using it for their many Joomla! clients who have completely different websites. Sharing the core Joomla! engine allows for developers to host, support and upgrade many Joomla! websites easily.
I recommend this low-cost component to everyone! There are many add-ons for it, so you can choose what you need.
Purchasing this system for my client has saved them so much money. They probably would not have been able to afford the development costs of engineering a system for their needs. But Jms saved the day!