Just spent an hour importing a huge catalog into Episerver, only to realize not a single resource file from the PIM has been imported? 

If your error log is full of something like the following:

Unable to create/update metadata for Resource ID: 126911. The provided content link does not have a value. Parameter name: contentLink

..and you're using a HierarchicalCatalogPartialRouter roughly like the one below - which would be common in an Episerver Commerce scenario. Using ContentReference.StartPage is the essential part.

var catalogRouter = new HierarchicalCatalogPartialRouter(
() =>  {       
    return contentLoader.Get(ContentReference.StartPage);


...the solution for you is very likely to add a wildcard host mapping in admin mode. Without it, Episerver will simply return an empty reference on ContentReference.StartPage (or SiteDefinition.Current.StartPage) in connector import context. Add one like shown below:

This wasn't all too easy to find out about, so I've submitted a pull request (You lack access if you get a broken link there. Ask inRiver for it if you want.) to improve the error logging in the connector. Hopefully it'll be included in future versions of the connector.