I thought I'd post this to settle any debate over the value of mastery for holy priests.
To begin with lets remind ourselves what Echo of Light does.
Echo of light adds a 6 sec HoT to any direct healing spell on the target. The healing done over these 6 seconds is equal to a certain percentage of the direct healing spell in consideration. The percentage itself depends on your mastery rating. Each point in mastery, that is every 100 mastery rating (because 1 mastery point = 100 mastery rating), increases the Echo of Light effect by 1.25%.
There is quite some dilemma about how to gem and enchant correctly for holy priests. I myself have seen a variety of gem configs on priests in my server, some of them amusing while others not so amusing. Here's what you should know before you spend that gold of yours on enchanting and gemming your holy priest.
Suppose you have X intellect. For the sake of ease, let X be your spellpower too, and let any arbitrary healing spell you cast generate X amount of direct healing. The question is whether gemming for mastery, as opposed to intellect is worth it.
It should be obvious to anyone who is slightly mathematically inclined that mastery being a % scaling will at some point outweigh the linear gains generated by increments in intellect.
Again, for convenience suppose we are considering choosing between +20 intellect versus +20 mastery rating.
Note that +20 mastery rating = +0.25% healing to the Echo of Light effect. This is because 20 mastery rating is one-fifth of a mastery point (reminder that 1 mastery point = 100 rating), and since one mastery point produces an extra 1.25% Echo of Light healing, therefore 20 mastery rating would do one fifth of 1.25% which is 0.25%.
So finally it boils down to calculating at what point of intellect are we indifferent between an increase of +20 intellect versus +20 mastery rating. Suppose before choosing between the two we have a mastery rating which gives Y% extra healing through Echo of Light.
If we go for +20 intellect, the new Echo of Light effect = (X+20)*Y/100
If we go for +20 mastery rating, the new Echo of Light effect = X(Y+0.25)/100
On equating these two quantities, we get
20Y = 0.25X
Now lets put some numbers to X and Y.
Suppose Y is 15, then the X that equates the above is 1200. This means that if your mastery is generating 15% Echo of Light healing, then for intellect levels greater than 1200 you will do better in pure healing output by investing in mastery than you would by investing in intellect. I say pure healing output because you do lose out on a larger mana pool, and hence a bit of mana regen through replenishment effects. Notwithstanding, lets put these numbers in context.
Usuallly, the X quantity will be larger than your intellect stat because you would have some spellpower bonus from your weapon. This even larger X tilts the balance in favour of +20 mastery even further. It should be also clear that X=1200 is a very low threshold going by cataclysm (and even WOTLK!) gear levels. Any semi-decently geared priest would have the X quantity in excess of 5000.
Gemming mastery is the way to go .. yellow slots should take +40 mastery, red ones should take +20 int +20 mastery, blue ones should take +20 spirit +20 mastery.
Enchant your gear for mastery. The results in healing output are pleasantly surprising.
Good luck, and thanks for reading !