I upgraded the rear shifter from XT to XTR a couple of weeks back now and am loving it! The difference between the two shifters is surprising. Compared to the light and definite action of the XTR shifter, the action of the XT shifter is like pushing the shift lever through sludge with indistinct shift clicks along the way.
Shimano refer to the magic as ‘Vivid Index’ which surprisingly is a pretty good description of what the XTR shifter action feels like. The lever action is light but with distinct positive clicks. Combined with the XTR Shadow Plus rear derailleur the shifting sure is uber slick.