If both ends of the communication are on the Microsoft stack, setting up SOAP webservices is straightforward. The tools basically take care of everything. In our case, we have a customer who exposed an ASP.NET webservice that we consume (data flows in both directions). Our production apps use VB.NET to hit the webservice, but I was able to easily write a python client for testing (by handrolling the SOAP messages).

My intuition is that SOAP starts becoming unpleasant if you have lots of parties involved and/or multiple platforms. We haven't hit that wall.