Mika Epstein is one of the foremost leaders in WordPress Multisite development, administration, and networking. During the WordCamp San Francisco 2013, she gave an excellent presentation called “Don’t Use WordPress Multisite” in which she argued that most single instance WordPress sites can do what you need multisite to do with a healthy use of plugins. Although Mika describes some scenarios where WordPress multisite can be useful, she presents 15 reasons why you should steer away from Multisite that should be carefully reviewed by many developers prior to building a multisite monster.
In addition to her WordCamp presentation, both of her books on WordPress Multisite, WordPress Multisite 101 and WordPress Multisite 110, are excellent introductions to learning multisite, optimizing your installation, and dealing with gotcha’s as a super admin. In fact, WordPress Multisite 110 should be required reading for all WordPress Multisite developers considering that it contains significant detail concerning the planning and considerations you need to make if you do decide you need to build a WordPress Multisite.