I feel that no matter how good the samples sound, it's depends mainly on the player's skill to make them sound convincingly realistic.

Knowing how the real-life instrument responds to the player, and the keyboard player having (or developing) the skills necessary to make the virtual instrument (whether in an arranger or as a VST) respond in a life-like manner, makes a big difference.

Some VST's and samples (Yamaha's SA/SA2 and Korg's DNC) have performance enhancers built in, but it still boils down to the virtuosity (and instrument knowledge) of the player.

Ian