We have just implemented our internal Live Communication Server, and it has changed my World. Up to now, most people in the company have been using MSN Messenger, and that has worked well. But LCS just makes the whole thing so much nicer.
Our company is small enough to have everyone in everyone else's contact list. This is becoming my new phone directory. The user presence information (online, busy, in meeting) details link up to your Outlook calendar, so it's now easier to determine if someone is available.
I've been using the application sharing feature a lot - even with a colleague that sits opposite me. Maybe we are just lazy, but it's handy to just share Microsoft Word from my desktop and talk him through the document as I scroll through it. The voice chat quality is also great, because it's using our internal network rather than going out to the cloud.
As for SharePoint - now EVERYONES user presence details appears through-out the web pages. Previously, that only worked if your MSN Messenger e-mail log-in matched your corporate account.