The .NET integrated classes have their merits.
But they have their shortcomings.
For example, DataContractJsonSerializer is not available in .NET 2.0, System.Web.Extensions needs admin rights to install it (in NET 2.0 - you can localcopy it, if you don't have a WebSite project) plus it doesn't work in SilverLight and WindowsPhone.
If you have a WebSite project, you need to copy the System.Web.Extensions assemblies to your project, and remove them from GAC afterwards, else VisualStudio doesn't understand it has to localcopy them.
But more importantly, if you work with pretty much any JavaScript library, e.g. SlickGrid (AJAX grid), you will stumble upon this valid JavaScript object (but it's invalid JSON, because fnFormatDate_DE is a function call and not text, it lacks the quotation marks):
FormatterCallback :
{
name : "DateFormatter_DE"
func: fnFormatDate_DE(val)
}
No chance to serialize this with any of the .NET integrated classes (because it's invalid JSON). Also, they fall short in terms of performance, availability in SilverLight, Windows Phone and WindowsRT. They are neither OpenSource nor MIT license. They have no support for indentation (human readable JSON), and they can't serialize DataTables, and they have problems with circular references. You can't handle serialization errors with them, can't serialize enums to their names, and you can't switch the date format (OK, this is not really a problem, because the MS date format is the only date format the safari crap understands [it doesn't undestand ISO]), and they don't serialize neither nHibernate nor Entity...
But most importantly, you won't want to switch your library or adjust project references if you go from .NET 2.0 to 4.0, you don't want to rewrite your code if you want to use some code in SilverLight/Windows Phone, and you don't want to be write a function to beautify JSON if you want to look whether you got the class right, and you won't want to write your own method to strip out quotation marks just because Microsoft's libraries can't handle invalid JSON.
Also, Microsoft's libraries have a low performance, and they can't serialize to BSON (for use with NoSQL databases like MongoDB).
So for all these reasons, you better choose NewtonSoft JSON (JSON.NET).
It's free and OpenSource (MIT license, not GPL).
There is a nice comparison matrix here:
http://james.newtonking.com/pages/json-net.aspx
