I have been using Joomla for just over 2 years and am currently managing just over 100 joomla sites, of these 5 also use community builder.
CB is quick and simple to install, works straight from the box and has a wide range of easy to install and use 3rd party modules and plugins.
I have never had a days downtime on any site using CB, and over the 5 sites have over 2000 registered users.
I have never had to use their support, so cannot comment on their abilities, this just shows how easy it is to install and use.
Thanks to everyone in the Joomla and CB development teams. I love you all.