We operate and manage several cloud infrastructure services both for ourselves and several client enterprises. Using several solutions for the management and deployment of virtualized resources, including VMware, Xenserver, and OpenStack, our Goto solution that we use internally is CloudStack.
CloudStack not only integrates well with the various forward facing customer billing and management solutions we use such as WHMCS, HostBill, and Blesta, but can manage virtualized resources directly and even works well sitting atop and management vSphere, vCloud Director, etc., and Xenserver - so many of the layers that only manage services under a particular virtualization technology can still be used directly or managed by CloudStack.
We highly recommend CloudStack as a solution for the management of enterprise resources in the cloud and use it everyday ourselves.
Unlike the freedom that OpenStack offers in Enterprise Infrastructure design, CloudStack is somewhat more rigid in the way you can deploy it, although almost all customization is still possible, it's intended to launch and use in particular ways that OpenStack leaves to the engineering team to determine for themselves.
Make sure all of your existing infrastructure management tools are working correctly (deploy, spin up, manage, snapshot, etc.) before layering CloudStack on top of them or having it manage your resources directly.
As you add layers of complexity on top of existing ones, the broken parts fly further when something isn't right, making it more difficult to ascertain particular problems.
Whether our clients are VPS hosting providers or operate private cloud infrastructure for their internal enterprise requirements, CloudStack provides solutions in an easy to implement and operate way that is simple to provide training for.
The learning curve for our customesr adopting CloudStack for the management of their cloud infrastructure is shallow, enabling our clients to come up to speed quickly without having to outsource their administrative functions any longer than necessary.