If you are looking for a simple tool which can read, edit, modify the xml content as well as exchange the xml content with other partners and machines, then I would say the best tools I found so far were Microsoft Infopath and Adobe LiveCycle Designer.
While one part of world is going towards browser based user-intefaces, their exists many, who still prefer a non-browser based plug-n-play tool which can do start their job while offline and finish it while online.
Many companies have requirements which is not letting them to have browser based document storage and exchange.
Both tool can be configured to use webservices. Email, xml validation, input validation, input driven events etc., were also available.
Before you start writing your own tool, its worth looking at them. MS Infopath cost around $200. Adobe LiveCycle Designer is $400.
Please note: This is my personal view and dont start blaming me for any issues.