We just converted over from a PBX to CM 2 months ago. We were told that we could have same line appearing on multiple phones, and you can easily configure that in CCM admin, BUT let me tell you there will be problems. It will work swell if all the devices that display the shared line do not have separate voice mail boxes. However if you have a device (phone) with voice mail and a shared line displayed on the phone which also has voice mail of its own, the system gets "confused" as to which mail box to send the message to. It doesnt matter which way you set the 'forward no answer' and 'forward busy' to, (believe me I tried all combinations) it will work sometimes and not others. In the end we solved the problem by removeing all instances of the shared lines. When Cisco was asked about this the answer did make sense, it was 'if you are setting up a shared line, the point is so that the call can be answered by a human. And if that is the point, there is no reasosn for voice mailbox to be configured for that extension, right?' True....
The bottom line is that IP telephony is NOT the same as a PBX system, although it does a decent job of emulating it.
You will be extremely happy with the added features and benefits (totally free phone calls inside your network, complete traceability of every button pushed and calls placed/received, see the company directory online, manage users, devices and speed dials, extensions at HOME via VPN connection to CCM) but you may miss some of the old features that you are used to in a traditional PBX system.