What are the drawbacks with using UUID or GUID as a primary key?

I would like to build a distributed system. I need to store data in databases and it would be helpful to use an UUID or a GUID as a primary key on some tables. I assume it's a drawbacks with this design since the UUID/GUID is quite large and they are almost random. The alternative is to use an auto-incremented INT or LONG. What are the drawbacks with using UUID or GUID as a primary key for my tables? I will probably use Derby/JavaDB (on the clients) and PostgreSQL (on the server) as DBMS.