UA in raids is great. On long boss fights, it's awesome. It does more DPS than locks with Ruin. At least from my experience (when I had it).
And of course, UA + CoT on priests/pallys in PvP is always fun.
That said, I currently have ruin. The reason is because I am doing more heroic/5-man than raids and I want the burst that ruin provides. As my guild progresses with raids, etc. I will probably pick up UA again -- but right now I want burst damage and Ruin helps to give me that.
This spreadsheet is pretty helpful with your question, too.
(another) DPS Spreadsheet