The good thing about the combo jack is that it works with the hundreds of 3.5mm phone headsets which a lot of people already carry. There is a wide range of choice at all cost and quality levels (newer Blackberry, iPhone, etc. headsets).
On the other hand, anyone with an exisiting separate mic and headphones will have a problem. A simple adapter solves this problem. See the threads in "Related Links" to the right for examples of adapters which community members are using.