I've seen armies with two doing well in competitive play. They are good units, and their meltagun is really nice, as they can get close easily. With their drones, they can be hard to take down until you commit them.
I'd probably not take too many, as they are good tools, but not an ubiquitous unit that can do everything. But, they are something I seriously consider when playing against Tau in objective based missions. Same with stealth suits, they are super annoying to deal with.
If you're just playing kill points, they might not give you the same return on investment, but if you're playing objective based missions (like ITC, or ETC missions), they are good.