Now that you have a few disco priests raiding, I thought I'd throw a bit of advice up. It's nothing that you can't learn by doing some maths or reading mmo champ, but I know that's not everyone's cup of tea.
This is not a beginner's guide, and I won't explain all the terminology...
Some priests make a lot of use of atonement, some less so. I'm more in the former camp, for a few reasons. Firstly, atonement healing is really effecient, as you're dipping into any DPS-increasing mechanics (think Elegon, Un'sok, Sha) as well as debuffs on the boss (CoE and the like) to directly increase your HpM (healing per mana).
Secondly, atonement heals are smart, and so will heal the person who needs it most (within 40 yards of the boss, which is the whole raid on most fights). This is particularly important with Penance; cast as a heal, you will often overheal with the final tick. Cast as an offensive spell, each of the three ticks count as three seperate atonement heals, and can hit different people.
Finally, you'll contribute a decent chunk of damage to the raid, which is generaly good, but particularly important when you're up against a tight enrage timer.
First of all - glyph it! Disco priests aren't particularly mobile. PW:S is powerful, but once you've put weakened soul on someone, all you can really do is renew them if you're, for instance, dodging tornadoes on Zor'lok. Being able to cast Penance on the move is therefore vital to being useful in many boss fights.
Penance should usually be used as an offensive spell, for the reasons outlined above; which are that it'll heal for more that way, and it can heal more than one person. The only reasons to cast it as a direct heal are a) to quickly stack Grace on someone (usually a tank) or b) when you need the whole spell to hit a particular target for some complicated reason.
It's very important to maintain Grace on the tanks, if you're going to be healing them for significant amounts. This is easily done by chucking them a Heal every few seconds, or using Penance as a direct heal.
To maximise the benefit of Rapture, you should get a timer active and keep track of the ICD. Basically, you should be applying PW:S to the MT every 15 seconds or so, and only using it on anyone else if they are in very immediate danger of death, or you have no other option (ie. you're moving and Penance is on CD).
SS is the reason that the number of disco priests raiding has increase massively in the last few months. By stacking quite a lot of mastery and timing it well, you can shield your whole raid for significant amounts. It's a very powerful ability, and should be used very often when you know large amounts of damage are incoming. It is particularly powerful on:
Stone Guard, Feng, Spirit Kings, Elegon, Will, Zorlok, Tayak, Meljarak, Unsok, Shek'zeer, Tsulong, Lei Shi (though its a bit random) and Sha of Fear. You can work out which mechanic it's best for yourself
Basically, use it when at leasy 60% of the shielding will get used. Because of its interaction with mastery, that is roughly the point at which it's more effecient than healing. It also buys you and the other healers time to heal people up, and will often cover up small errors in positioning etc.
Quite a lot of disco priests, especially those new to the class, are going all out to stack mastery to very high levels. This is a valid strategy, IF at least 60% of all your healing comes from Spirit Shell and PoH. It will certainly not remain a valid strategy for very long, because 5.2 is bringing a big nerf to mastery-stackers; not only will SS no longer benefit, but PoH will no longer automatically proc DA.
My opinion is that mastery stacking looks great on healing meters, but is not the most effective way to contribute to keeping your raid alive. Disco priests have a lot of tools at their disposal, and stacking mastery is gimping all your atonement and single-target healing in exchange for big burst AoE shielding that will be largely overheals unless used very expertly.
My advice is to aim to balance your three secondary stats, with a slight bias towards crit and mastery over haste (up to 20% bias). Of the top two, prioritise crit slightly if you have to choose. Don't balance the percentages, balance the rating numbers. Do this BEFORE taking into account the buffs you'll be getting in raiding circumstances (the most significant of which is 3000 mastery, which will near double your mastery rating).
With balanced stats, you can adapt your healing depending on the raid encounter and who you're healing with without gimping yourself.