Exchange 2013 product and especially CU update quality is really, really poor. Many admins are afraid to install CU updates fresh when they arrive since there are many cases where it might corrupt entire Exchange installation and lead to severe outage and disaster recovery scenarios. The situation now is that one needs to wait 2-3 months before applying CU's to see if someone has broken their environment and blogged about fixes and tips to prevent that from happening. Currently waiting for CU3 to a known issue with Health Manager after spending hours and hours with Premier support. Can't really recommend upgrading to Exchange 2013 to any customer at the moment.