We have built solutions using InDesign Server since it came out, and before that we were building solutions based on InDesign desktop for 5 years. So we know the XMPie space pretty well.
XMPie is a really well-built program, that to me has three main benefits:
XMPie is salvation for the designer at a mail house: they can bypass Programming entirely and set up their own “campaign” based on new InDesign/data input from a client.
Yet these days, we never meet such a designer. We meet enterprise clients, who consider themselves very special and do things a very special way. XMPie invariably meets their needs 40-80% of the way, but the other 20-60% can take a supreme effort. So we need to request extensibility from XMPie, and in many case fuse together an XMPie workflow with a very non-XMPie workflow. XMPie may be sick of my requests, but they have given us more and more extensibility over time.
InDesign Server has to its advantage complete flexibility, but if you use InDesign Server alone you have to build several features that are pre-existing with XMPie. It really depends on specific workflows/document types/staff whether XMPie is the right fit.