If you're into dynamic languages, maybe you're like me and you think of mixins when you hear the phrase "partial classes." Wouldn't be the first time Microsoft gave old technology a new name and called it innovation, right?
Unfortunately for C# developers, partial classes have nothing to do with refactoring common behavior into a single reusable code fragment. All it is is a way to break a class up into multiple files for the benefit of code generation tools like Studio 2005's GUI designer. This is something that could easily be done with annotations/decorators/attributes (Java/Python/C# terms for pretty-close-to-the-same-thing). In fact, this is the approach the NetBeans Java IDE has been taking for years with their Swing designer (called "guarded [blocks|areas|code|chunks]"); it works fine.
My friend says that he could see this being useful to other, third party code generators, but I have a hard time seeing that carrying enough weight with Microsoft to actually spawn a new language feature, even if it were the only way to accomplish IDE support for code generation, which it isn't.
Some of the more rabid .NET fanboys have suggested that this will be cool because it lets you physically split a class among different developers. This is a lousy idea on several levels. First, if your classes are that large you probably need to rethink your design. Second, if you really do have an ironclad reason for this monster class, remember that one of the main goals of OO design is that someone who only wants to consume your class only needs to know the public API. People advocating splitting classes up into multiple human-edited files are forgetting that the other side of that coin is, the implementors of this class do need to know everything about it, or you'll be in a situation far worse than ordinary conflict merging.
Maybe I'm missing something. It really does baffle me that the C# team would go to the trouble (and it is trouble) to add a feature with only one real use case, and a flimsy one at that. Most of C# design I can see good reasons behind, if you take as a starting point "Like Java, only with the benefit of several years of hindsight. Oh yeah, and Sun doesn't own it." Some of the decisions are arguable but this one makes me scratch my head.