Well, having tried the free Xceed data grid for a while, and not being fully pleased, I’m going to give Infragistics control a try. They’ve released the XamDataGrid for free (link currently broken?), and also put out a promotional on their full suite for MSDN subscribers for only $199 a seat.
My biggest concerns at this point will be their support responsiveness, especially as concerns their forums, as I’ve struggled with them before. Their controls are generally among the best looking, but I’ve often found it a struggle to get their controls to behave exactly as desired. First glance at the WPF suite seems to show significant improvement in this regards, but we’ll see how they respond to specific challenges.