Last year they changed 10 installs to 5 installs for the same price. I'm blown away by their technical support though, in fact, it's the best I've experienced. Check carefully before you sign up and see what all you get for the price since they change their packages often it seems. I've stayed with them since I've had horrid experiences with tech support from other companies in the past, and it's not a fun thing when you spend all night long trying to do all of the research yourself because no one wanted to help you. With that said, I've certainly saved a lot of time and hassle from using WP Engine. Time is priceless and I'll keep spending whatever it takes (within reason) to keep having superior customer service.
WP Engine also offers a number of premium security features. The WordPress core is automatically updated to the latest version for you. WP Engine thoroughly tests any major core updates before upgrading their customers. They have a proprietary intrusion detection and prevention system to block any DDoS attacks, brute force attacks, JavaScript/SQL-injection attacks and more.