Only with respects to the Force itself: take the Force away from a Jedi or a Sith, and it's very much like losing a limb. Your senses aren't what they used to be, your co-ordination is off, and you lack that intuition and insight that would once have made you a highly reactive, very dangerous and well-prepared individual.
Now, Grievous did prove that a non-Force User can wield a lightsaber, and even to deadly effect, but remember, he was cybernetically enhanced, so his sole tool in being able to kill Jedi was in that he possessed a mechanically-derived speed that, as we all know, moves well past the capabilities of flesh and blood sentient beings. That observed, he lacked the Force and the intuition it offers those who are sensitive to it - thus was only able to analyse a situation based upon perceptual thought rather than special insight, so he literally could only deal with the cards dealt as he saw them. A Jedi or Sith have that little touch of precognition that allow them to react to a situation and also to alter their reaction to it: how you'd respond to a guy throwing a ball at the back of your head is different if you know he's going to do it just beforehand (even a split-second of precognition makes a big difference).
More to the point, lightsabers are tricky things to wield, because the weight is all in the hilt. It can be used by someone not attuned to the Force (though as "I, Jedi" indicated, a lightsaber constructed by a Force User is far more than a mere weapon, and is bonded to them in ways beyond the physical). You could train to use one, and even do it well, but it would require a considerable level of finesse, and would very much be a different art to, say, wielding a sword or vibroblade, because even with the same techniques of attack, defense etc (parries, thrusts, slashes, blocks etc), the very nature and handling of the lightsaber would make it a very tricky weapon for anyone not blessed with extrasensory perception to master effectively.