We\'re doing a mapping process from an XML file generated by a legacy system to EDI 834/837 files. We have BizTalk 2010 and are using the Microsoft built in EDI schemas.
My experience with BizTalk maps is that things that are very simple to do with XSLT can be very complex with maps.
For good counter-examples of BizTalk maps, look at the book "Pro Mapping in BizTalk Server 2009". The book has some examples of very complex things you can achieve with BizTalk maps, but the downside to it is that in fact they have hidden all the complexity in scripting functoids. Therefore, the maps are not visual at all anymore (they don't even have links between nodes to provide at least hints to deduce what the map is doing).
XSLT can be more visual than a map, since you can see the resulting XML in the XSLT (keep in mind that "text" does not imply "not visual" - if you are transforming between text formats, then a natural way to visualize the transformation is by looking at text)
BizTalk maps can be used for very simple mappings, where you are essentially copying a set of properties from one structure to another structure with the same properties. However, as soon as you have to map a structure to another different structure, you quickly get something that's hard to write AND hard to read/understand.