What GUID provide? http://jasegroup.com/do-you-really-need-a-guid-database-design/

Why primary key are cluster key? http://www.thescripts.com/forum/thread81459.html

Management key yourself but not letting framework to do that for you - http://www.onjava.com/lpt/a/6718

Comparing compound key and single key  - http://vladmihalcea.com/2014/06/20/database-primary-key-flavors/

Some online resource discuss about GUID:

http://www.codinghorror.com/blog/archives/000817.html 
http://www.codinghorror.com/blog/archives/000399.html 
http://groups.google.com/group/microsoft.public.sqlserver.programming/msg/6d61dbf80d6f0fb6?hl=en&lr=&ie=UTF-8&oe=UTF-8&rnum=14 http://www.informit.com/articles/printerfriendly.asp?p=25862&rl=1 
http://krow.livejournal.com/497839.html 
http://en.wikipedia.org/wiki/Globally_Unique_Identifier 
http://databases.aspfaq.com/database/what-should-i-choose-for-my-primary-key.html 
http://blog.maxindelicato.com/2008/12/how-to-organize-a-database-tables-keys-for-scalability.html