I never use GUID's as primary keys.  However, I have found them very useful in the following scenario.  Because the generated numbers are essentially random and unique, whenever I need to randomly select a number of records from a group, I assign a GUID to each row.  Assume a table with 1000 records and I want to select 100 at random.  I can assign a GUID to each row, and then select the top 100 ordering by the GUID.  Because the generated GUID's are random, I know that the rows that get selected will be sufficiently randomized.  This has worked well for me in the past.

Jonathan