Depends.
Both my current Modern games and
M&M game and my previous Modern,
M&M, and
Sidewinder games are 100% homebrew, as was my threepointoh D&D game.
However, if I run another D&D game, I'm pretty sure I'll use the
Wilderlands of High Fantasy and
Ghostwalk and create just the adventures. First, I love those settings, (b), I'm not as excited about the idea of creating my own fantasy campaign-setting as I once was, and 3., I'd rather devote my creative energies to more Modern and Past games for which I have far more plans and schemes than time.
My answer, by the way, was "Mostly homebrew..." in expectation of someday running a
Wilderlands game, but I'm still the ultimate GM, IMHO...
