We have previously used the RTC API via COM and found it to quite helpful. However, for an upcoming product, we will be taking advantage of the Mircosoft UC AJAX API.
The RTC COM API were useful, but did not scale appropriately for high volume server side applications. At one point, we created an abstraction layer to wrap both the RTC COM API and the Sametime COM APIs. Our Instant Agent Navigator is built using this abstaction layer and supports both OCS and Sametime.