Former lets break'ing out, latter doesn't.

do { ... } while (0) makes your macro behave like a statement (i.e. the semicolon is mandatory)

Yes. I don't include the trailing semicolon in macro definitions, because I expect the macro to be followed by them:

    MACRO(foo);

({ ... }) does this too. (Is this GCC-specific?)

That is a gcc extension. The construct as a whole has the value of the last statement executed within. Usually an inline function is preferable since it achieves the same thing using only standard syntax.