MS Visual C++, as well as several other compilers, now offer non-standard keywords that control the inline expansion of a function, in addition to the standard inline keyword. What are the uses of the non-standard keywords? First, let's review the semantics of inline. The decision whether a function declared inline is actually inline-expanded is left to the sole discretion of the compiler. Thus, inline is only a recommendation. For example, the compiler may refuse to inline functions that have loops or functions that are simply too large, even if they are declared inline.
By contrast, the non-standard keyword __forceinline overrides the compiler's heuristics and forces it to inline a function that it would normally refuse to inline. I'm not sure I can think of a good reason to use __forceinline as it may cause a bloated executable file and a reduced instruction-cache hit. Furthermore, under extreme conditions, the compiler may not respect the __forceinline request either. So, in general, you should stick to good old inline. inline is portable and it enables the compiler to "do the right thing".

__forceinline should be used only when all the following conditions hold true: inline is not respected by the compiler, your code is not to be ported to other platforms, and you are certain that inlining really boosts performance (and of course, you truly need that performance boost).