I recently went deep into discipline just to try it out and to gain the spirit buff for my raid group.
This was my spec: http://www.wowhead.com/?talent=bVcbuhhtrxoifRtfxoc
Here are the pro’s and con’s as I saw being a discipline specced healing priest.
PROS
Penance. Penance heals the target three times in quick succession and replaces/exceeds the need to use prayer of mending as an instant quick heal. You just have to be careful that you are facing the target you want to heal as penance is a directional spell. (update 3.08: Penance is no longer directional,you will automatically pivot on the spot to face your target)
Preventing Damage. You prevent damage to more than heal, your targets. Thanks to the Divine Aegis talent which protects your target from 30% damage every time your heals crit on them. And also thanks to the Grace talent which procs everytime you use flash heal, greater heal or penance. It protects your target from an extra 1% damage and also increases the healing you do to them by 2%.
Improved Divine Spirit. Very handy when your raid group doesn’t have another class that produces the same effect.
No Issue with Mana Regen. I found that boss fights where I was usually running out of mana on as a holy priest, I had no troublewith it as discipline. Apart from having the talent to increase your intellect by 15%, you also get the talent Rapture which gives you 2.5% of your maximum mana back each time you heal with greater heal, flash heal or penance.
Yes, you can still heal heroics! I thought I would put discipline to the test and take a bunch of guinea pigs down with me by trying to heal Halls of Lightning on heroic mode. I had around 1700 heals at the time (self buffed) and we all managed to survive without any major hiccups even through Loken. I did rely a little more than usual on Prayer of Mending to help keep up raid healing.
DPS. I was doing the Sons of Hodir dailies and found it delightfully quicker to complete as discipline spec compared with holy spec. I found I was also sporadically doing around 400 dps on top of healing in a naxxramas raid when there were three healers (including myself). Also handy in heroics when the tank isn’t taking too much damage.
CONS
Poor Raid Healing. Sadly as discipline, you are not very effective as a raid healer since you no longer have the instant cast circle of healing spell.
Pain Suppression is a poor substitute for Guardian Spirit. It just doesn’t quite have the same effect and while it is very useful in similar situations it’s just not as, lets say… stress-relieving(!) as guardian spirit is!
No Spirit of Redemption. Gone are the “after death” heals that make or break the fight. I didn’t realise how much I use the spirit as a time when I can freely cast and hot up the raid while throwing out prayer of mending at the last second in the hopes that everyone stays alive while you are dead.
Bottom of the Heal Meters. If you don’t care about anything but getting to the top of the healing meters, then you won’t enjoy discipline healing. While you don’t necessarily come in at the bottom, you aren’t anywhere near the top either. As I mentioned in the pros section above, discipline healing is more about shielding and protecting your targets rather that outright healing them. If you add up all the damage you prevented on top of your healing, then maybe you’d be back up the top of the heal meters, but unfortunately that’s not included in the ‘effective heals’ in the add-on Recount, so it just looks like you didn’t do much healing.