It's a personal thing, however those who prefer them must be a small minority or SRAM would not have developed pod shifters to compete with Shimano. I understand the SRAM thumb shifter pods are superior to Shimao XTR (plus we don't all like brake levers to do the shifting) and in my area get rave reviews.
The cheap-line of grip-shifts are lighter and work better than the cheap line of Shimano which is really clunky. I ran ito that when I bought "kids" bikes for the grand children. I had to throw away the Shimano stuff on the G son's bike, but I kept the grip-shifts on the G daughter's; they were bottom of the line, but worked very well.
Al