As I see this topic recurring again, I wanted to humbly offer up the old entry To GUID or not to GUID In Your Databases.

Consider that simple joins are continually held as one of the primary weakpoints of many web scale implementations. Add the complexity of joining GUIDs (not even considering the lower data density, slower inserts, etc). It can be brutal.

This is one of those topics that is often incorrectly classified as premature optimization. It isn't. Your data structures will often live longer than any other element of your project. That is simple truth, demonstrated by millions of projects across millions of organizations.