My Tau list so far has included a Skyray. The clarifications on Seeker Missiles made them seem like a tempting include in my eventual Adepticon list. They bring along a pair of mobile marker lights which also seamed like a good enough reason to give them a go. Third, the Skyray box includes the parts to make a Hammerhead, so I could still equip the hull with a rail-gun if it didn't work out.
The first couple of games I used the Skyray it preformed fairly well. It provided more benefit than my Hammerhead or Broadsides. Having mobile marker-lights upgraded to hit on 3+ was a solid addition to the list, and it was devastating to most targets to launch a Marcross Missile Massacre from the Skyray. For most of these battles, I was facing lightly armored foes. Most of the available armored targets could be adequately handled by battle-suit missile pods. In those games my st 10 rail-guns where superfluous. Their low rate of fire meant that they didn't have quite the effect of markerlight boosted st 7 missile pods and st 8 seekers.
The shine wore off the Skyray when I played against Necrons in a Dawn of war game. The first issue that arose was that my Skyray was stuck in support mode while my Pathfinders moved into position early game. This meant that I never had the opportunity to fire any seekers. The second issue was that my missile pods where fantastically useless against Monoliths and only slightly less useless against Quantum Shielding.
This left the whole anti-armor duty for my army onto my two Boadsides and single Hammerhead. I was rolling pretty badly for my rail-guns, and only managed to immobilize one Monolith with them. I was wishing for a third rail-gun equipped unit the whole game.
This was only my 1500 point list. My full 1850 Adepticon list adds in two Piranhas to assist against heavy armor, but that may not be good enough. I'm coming around to the standard reasoning that-no matter how good the Skyray is-it's not as good as anything carrying a rail-gun.
Those two marker-lights the Skyray provides are hard to give up, but I think in the long run having at least one more rail-gun will be more useful. I would like to add in another squad of two Broadsides in place of the Skyray, but practical considerations will probably lead to using an additional Hammerhead instead. If I can get the army completed and painted ahead of schedule, I may convert two additional Broadsides. As it is, I'm not sure I'll have the time to do that.