What is SQL Server Parallel Data Warehouse?
It isn't inexpensive. I don't know the price tag but you are talking large capital expense measured in hundreds of thousands of dollars. It is an enterprise scale solution that is optimized for large scale enterprise data warehouses. As such, you may find it tough to get a great answer here other than things like the link I provide below.

You said you already did the research, so I won't provide you too many links other than the this one which does a great job explaining PDW.

If you have the budget and enterprise scale that requires PDW, you would be much better off talking to Microsoft and asking them for discussions with reference customers. This right now isn't deployed en masse, so you will get individualized attention if you are in the market for a massively parallel data warehousing appliance.