We did a proof of concept with OCS last fall and winter. It was proven to be not-quite-ready for prime time. For instance, when someone called my phone number, if I failed to answer it was to transfer to our regular VOIP system to allow the caller to leave a voice message. Often it would fail to transfer. Quality of the sound was often nearly unacceptable as well.
So stated, we do use the IM part of the system, about 100 of us, and it seems to work well. There are a few anomolies, such as having to log in each morning, but that’s likely just a result of the way we lay policies on our workstations to keep them safe. I do like the ability to know if someone is actually at their desk or not before walking upstairs to chat!