What are the advantages and disadvantages of using the GAC?

Question

And on top of that, are there cases where one has to use the global assembly cache or where one cannot use it?

Answer 1

The GAC runs with Full Trust and can be used by applications outside of your Web App. For example, Timer Jobs in Sharepoint HAVE to be in the GAC because the sptimer service is a separate process.

The "Full Trust" Part is also a possible source for security issues. Sure, you can work with Code Access Security, but I do not see too many Assemblies using CAS unfortunately :( The /bin Folder can be locked down to Medium which is normally fine.

Daniel Larson has a post on CAS as well which details the differences a bit more.