I've been using, and involved in deployments of Cisco Call Manger dating back to 2005. The greatest compliment that I can give the solution is that the platform stability is "rock-solid" and the solution is extremely dependable. Personally, I've never engaged with anyone that has Cisco Call Manager that isn't pleased with the solution.
I believe that Cisco is still playing catch-up in a few software areas, and you're required to purchase some additional 3rd party tools to perform basic functions such as detailed call reporting. However, I do feel that Cisco continues to close that software gap, in some areas, between them and their competitors. The biggest challenge is the complexity of the overall management of the platform. Once you're used to it, managing Call Manger is easy, but the learning curve to get there is steeper than other solutions.
Personally, I feel that it's the platform of choice, but you need to consider how it will be administered and supported internally within your organization. The initial complexity of the solution can be overwhelming if you're not prepared.
Other than the obvious basic dial-tone, we use it for it's soft-phone capabilities, single number reach, instant messaging, call control and flows, and unified messaging. It's a great help to our mobile workforce and users that migrate between our 30+ locations.