Apparently CloudStack "lost" to OpenStack ( see: http://www.infoworld.com/article/2608995/openstack/cloudstac... ). I've also heard this sentiment in my admittedly openstack biased circles.

That said, I'm not suggesting it should not be considered.

It lost out in getting vendor support from lots of different big vendors. OTOH, many hosting environments find it more useful. (It used to be a Citrix product which might explain the more unified architecture it has).

There are case studies also: https://cwiki.apache.org/confluence/display/CLOUDSTACK/Case+...

It looks like they have a pretty good user list going:

http://cloudstack.apache.org/users.html