Wraithguard are infinitely awesome at assault. It takes very special units to be able to deal with them quickly and easily - either poisoned melee, rending, or monstrous creatures seem to be the only thing that can destroy a full unit of them quickly. This makes them excellent at grabbing midfield objectives against an assaulty army.
While I haven't seen much in the way of Tau armies at my FLGS, I am certain they are coming and I am concerned with how I will handle all the longer range S5-7 shooting against my two 10-man Wraithguard squads. At first I thought breaking them in half and sticking them in Wave Serpents, but I have my doubts about how long those would survive, too.
If Tau are the paper to the Wraithguard rock, what else might be?