Vespid Stingwings have been one of those units that has intrigued me since the new codex was released. The insectoid aliens come equipped with a nasty S5 AP3 neuron blaster and have the ability to leap from cover to cover due to their wings and skilled flier special rule. The unit is hampered by a few surprising disabilities. While having a short range gun, the unit can't survive any reprisals by the enemy due it's light armor of 5+. Most basic weapons in the game have AP5 or better, meaning you'd better choose your targets wisely. On top of that Vespid are poor shots and need markerlight support to be effective.
I'd like to really see this unit come into its own. I can see the unit being very useful held in reserve and used a counter attack unit on any enemy forces that have broken your lines. Unfortunately though, with Mech lists being all the rage these days and Vespids having no real way of dealing with that, you're going to have get your opponents infantry on foot before Vespid can start causing damage.
Being metal models makes them more expensive to purchase so this stands to reason why Vespids are so rarely seen in Tau Cadres. My prediction is that once a new Tau codex comes out in a few years, Vespids are going to get a huge boost, maybe a tougher armor save and longer range on their blasters.
If there are Tau readers out there, who use vespid, I'd love to hear how you use them, in what squad sizes and which units they work well with.