How do the likely/unlikely macros in the Linux kernel work and what is their benefit?

I've been digging through some parts of the Linux kernel, and found calls like this:

if (unlikely(fd < 0)){    /* Do something */}

or

if (likely(!err)){    /* Do something */}

I've found the definition of them:

#define likely(x)       __builtin_expect((x),1)#define unlikely(x)     __builtin_expect((x),0)

I know that they are for optimization, but how do they work? And how much performance/size decrease can be expected from using them? And is it worth the hassle (and losing the portability probably) at least in bottleneck code (in userspace, of course).