Your certainly right about the YMMV - for me the chemistry makes all the difference between a mediocre and mind-blowing time.  While everyone can have an off night, I'm sorry to hear your experience wasn't even better.  So many variables, from the ladies performance, your performance, mood, chemistry, likes, dislikes, etc. - thats one of the things that keeps it so addictive.
But from my perspective? I'm smiling big (and worn out!) tonite - she rocks my world.

 -- Deep (beats two hours in the gym!) Heat