What are the best practices for using a GUID as a primary key, specifically regarding performance?

A GUID as the clustering key isn’t optimal, since due to its randomness, it will lead to massive page and index fragmentation and to generally bad performance. On the other hand in this distributed database world, it is absolutely the best way to merge distributed data. You can fire and forget merge tables without all the worry of ints matching across joined tables. GUIDs joins can be copied without any worry.