I must say, I'm surprised at the overall popularity of the Tau in this forum. I remember when they came out a lot of players were "OMG Tau is TEH GHEY!!!11! 40K IS RUINED 4EVER!!!"
... and I was among them. The Tau were just... too different, too anime-ish, too "Warhammer 40K: The Next Generation".
But I must admit their fluff and battlefield performance ultimately won me over, even though a lot of people went so far as to quit 40K in protest at the time. I especially like how the Kroot secretly despise the Tau and only joined them for the noms.
Point being, I do think their popularity is disproportionate to their influence in the overall 40K universe (the only ones I can think that have a less impact on the universe are the Dark Eldar), but they are a fun army to take on, and I believe we are witnessing the first steps in the Tau universe that show us that even their "For the Lesser Evil" philosophy is starting to collapse within itself.