Holy as a healer was pretty good. Perhaps not quite as a good as a Druid, but still pretty good. I'm not sure what all of the good comps for it are in TBC, but it's likely you can't go wrong with a Warrior or Rogue partner in 2's. 3's on the other hand, I imagine it fits really well into most double dps+healer comp. The best thing about TBC Holy, though, is the Shockadin spec. The burst damage output the Shockadin was capable of, in combination with its strong healing capability (since it is based in the Holy spec, of course) was called OP by many. No idea how viable a Shockadin spec is in arena, but it is bound to be loads of fun in PvP.