We have been using Microsoft On Line Services for a while now and as a result we have been able to shut down 8 servers in our server room. In addition to saving energy, we didn't skip a beat in moving from premise based servers to Microsoft's servers located in Washington State.
We were concerned that we might experience latency related issues but services has been stable.
We are the only locally certified MS voice partner in Hawaii so have been able to compare and contrast MS server based Office Communicator Server with their on line version and have found only minor feature differences.
You can use Exchange, Sharepoint, Live Meeting and Communicator for $10 a person per month. If you have a microsoft infrastructure, you should consider.