Whether you`re running a small business and want to host your several projects, or run massive organization with hundreds of sites, wordpress multiuser is the solution for you.
Pros:

• Centralized management system
• Easy upgrades…Imagine you have 14 separate wordpress sites. Keeping in mind how quickly wordpress evolves you`ll have to upgrade 14 sites every few months, which means backups, downtime on each site. How about a hundred sites? When you upgrade a WordPress MultiUser all sites in the network upgrade at the same time, which is a massive time saver.
• A lot of free plugins and themes
• The content from one site is completely isolated from the others

Cons:

• Eats a lot of server resources
• One major point against WordPress MultiUser is that if something fails all sites on the network will be down…I`ve come across these kinds of situations numerous times in my work. I`ve seen how a bad wordpress theme takes down a network of several hundred sites for days, before we find the problem.

Hint:

Many people use WPMU for link building, which requires the addresses of all sites on the network(used for link-building) to be different. Mapping the domains by itself is not a problem, but if you need them on different addressess you MUST have a VPS, so if you can only afford a shared hosting this solution won`t be appropriate for you.