I very much prefer it when I at least get the believable impression that she's enjoying it too. To use a moderately extreme comparison, if as I empty my balls into a woman's mouth she sticks her tongue out, opens her lips as wide as possible, stares me full in the eyes and has the look that says "mmm, more, keep it coming, I love it, thank you", I find that my own pleasure is heightened. If instead she screws her face up like a bulldog chewing a lemon, I find that my excitement dwindles.