And on top of that, are there cases where one has to use the global assembly cache or where one cannot use it?
So to sum it all, start simple and if you later see major performance gains if you put your assemblies into GAC and NGEN them, go for it, otherwise don't bother. GAC is more suitable for frameworks where there is expectation for library to be shared among more applications, in 99% of cases, you don't need it.
In all my life, I have had maybe one application where I had to put an assembly in the GAC, simply because these assemblies were part of a framework that a number of applications would use it, and it seemed right to put them into the GAC.
I think one of the biggest advantages of using the GAC is that you can have multiple versions of the same assembly registered and available to your applications. Personally, i don't like how it restricts movement from machine to machine (i don't like having to say, check out source on a new VPC and go through a bunch of steps to get it running because I have to register stuff in the GAC)
Advantage:
Disadvantage:
Recommendation: Leave the GAC to MS and friends. The gigabyte is very cheap now.
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.
If you're shipping a reusable library consisting of multiple assemblies, but only few of them form a facade, you can consider installing the assemblies into GAC, if the package is installed to developer's PCs.
Imagine, you ship 6 assemblies, and only one of these 6 assemblies contains a facade - i.e. other 5 are used only by the facade itself. You ship:
Developers using your project would like to reference just MyProduct.Facade.dll in their own projects. But when their project runs, it must be able to load all the assemblies it references - recursively. How this can be achieved? In general, they must be available either in Bin folder, on in GAC:
Note: last option doesn't make you to do the same while shipping the project to production PCs. You can either ship all the assemblies within Bin folder, or install them into GAC - all depends all your wish.
So the solution described shows the advantage of putting third-party assemblies into GAC during the development. It doesn't related to production.
As you may find, installation into GAC is mainly intended to solve the problem of location of required assemblies (dependencies). If an assembly is installed into GAC, you may consider it exists "nearby" any application. It's like adding path to .exe to your PATH variable, but in "managed way". - of course, this is rather simplified description ;)