There is considerably more capacity, features and easy of use in an IP solution. I do not intend to direct my voice traffic over the internet, still using a T1, at least initially. The IP solution is integrated with Outlook, Offers users PC control of the phone, MAC are easier, the phones features follow you if you wish and the system is easier to manage. At least those are my reasons for making hte switch. Some of these things can be done with traditional TDM PBX's but it is not as easy to do.
Shoretel offers a complete solution that is receiving very good reviews. It is more expensive than some of the other solutions available but it also seems to be more robust and mature a solution.
