I've seen Boon of Mutation cast once... sucessfully... turning my Eldrad into a spawn. I won the game , but morally he destroyed me!
I had a friend who for a short period of time obsess with Boon of Mutation on how cool it is (ignoring it's reliability) and he would run lists with Daemon Princes, Lords of Change, and Heralds of Tzeentch (changing how he runs each game) to maximize the number he got. Quite a few games I used armies like eldar and guard with low toughnesses and he rarily got it off, he did two times get my autarch which proved most helpful early game (sadly for him Eldrad's toughness 4 actually made a difference). Then the last tournament he used Daemons he ended up playing marines, turning both his characters and 3 marines (one or two being terminators) into spawn, easily winning the game.
(I know Boon of Mutation is TECHNICALLY not a psyker power)
In short, the ability is very unreliable but when it does work it can be huge, and plus, it's just plain fun, especially knowing a grot and a carnifex mutate into the same power model.
My favorite psyker power is Mind War, although it isn't exactly the most useful power, especially compared to Doom and Fortune, it is fun to do.
Anything from the Weirdboy psychic powers list.
Anything. Mmm weirdboyz.
The mouth of the Emperor shall meditate wisdom; from His tongue shall speak judgment