When it comes to performing massive data changes in our CA PPM environment, XOG is by far the best choice since it is fully supported and besides I'm already familiar with it. I can use a mail-merge to come up with the XMLs and then submit everything with four clicks using XOG & Query Bridge.
When our team needs to perform data requests, the Query API is by far the best choice. We recently needed to provide a list of names of all our CA PPM users along with their email addresses. This task was very easy to get done. We used XOG & Query Bridge to extract the information so that it could be used with a mail merge operation by another department.