I just finished a project that required heavy drag and drop functionality.  Basically, the task was to build a document editor composed of a large number of draggable elements.

Because I am a huge Infragistics fan, I tried...  I mean *really tried* to use the Infragistics Drag & Drop Framework as the foundation of it.  But it was difficult.  

My list of drag objects is constantly changing based on user filters, and the list of drop objects is constantly changing based on document paging.  This was very difficult to manage.  I couldn't figure out how to do garbage collection when the list changed, and I couldn't figure out an efficient way to use jQuery to pass in a list of objects.

Also, no matter what I tried, I couldn't get the user interface element that follows the mouse during a drag to represent right.

Finally, I gave up, and started looking for other solutions.  I stumbled onto the provider that is part of jQuery UI.  Everything was intuitive, and I accomplished in a couple of hours what I had struggled with using Infragistics for several days.

I want your product to be top tier, and I know that customer feedback can be valuable, and I hope that this feedback will give you some direction to help improve your product.