CentOS combines the best parts of an Enterprise-level Linux-based operating system (repackaging the Redhat Enterprise Linux Open-sourced pieces) and providing it free of charge without a subscription.
I've used CentOS since CentOS 5, and have been using it daily for servers running Samba and the like since 2012. I've since migrated those servers to CentOS 7, but the stability really can't be beat for the ease-of-use.
Support for products are also very solid for CentOS, due to the alignment and source-compatibility with RHEL. The same can be said for Debian, however I'm also a very big fan of the YUM package manager, which seems to help with dependency-resolution a fair bit better than apt.
EPEL is also a huge benefit to CentOS and RHEL. EPEL is software packages and extensions that provide updated/extended functionality over what is provided by default in CentOS. This allows being able to have newer versions of software installed without having to compile or download untrusted software from the Internet. EPEL forgoes some of the ultra-stable packaging CentOS is known for (sticking to older releases of software in order to not break compatibility), while gaining a very wide assortment of software that wouldn't otherwise be available.
Sometimes driver support is lacking, since the kernel versions being used tend to err to the 'oldstable' spectrum. Also, i'm not totally convinced that XFS/BTRFS is ready for the enterprise, as I've experienced issues with BTRFS snapshots in other Linux distributions (such as SLED). Sometimes as previously mentioned, packages are sometimes not updated as frequently as I'd like (this is done to preserve a cohesive environment and inter-operatibility between systems per release), but this is somewhat mitigated with EPEL packages.
I needed a stable system to provide Samba4 AD controllers for small business. I was using Fedora 14, when I realized support for various required systems weren't going to work (such as Quickbooks Enterprise), so I switched to CentOS, which has proven to be a very-wise choice. I virtualize several servers with XenServer, and have been very pleased at the ease-of-deployment and setup that is required with CentOS.