Its not fun when you ninja from random people in LFG. It just makes you a meanie.
However, if you do it in a guild run, take items you cant use and then starts to hearthstone while people are screaming and crying on vent (then post the video on youtube) its abit amusing. And the mean-factor of it will be overshadowed by the entertainment value.
Had people do this before and it was simply hilarious even if I needed one of the items myself.