I agree that disc pvp is not very viable right now. HOWEVER, it is very important that priests of every spec (no so much shadow though) understand that the way we do everything has changed a lot, between archangel and chakra and shadow orbs each spec need to read up on all of their changes.
I have done PvP with all three of the specs, because I love all of them and from my experience this is what I found:
Shadow: I think I can say that the shadow priest is moderately over powered right now. I have done multiple BGs with other classes, and I am seemingly more undergeared than them, and between our short cooldowns of psychic scream (which will not be so short in the next patch, it will be 30 seconds longer) and dispersion you should not have too much of a problem with doting and kiting your opponent. It does help to have a USE: SP or mastery trinket, as this really helps against hunters or rogues. Also, mana burn is your best friend, so if that is not currently part of your rotation against mana users you should def. start using that.
Disc: Unfortunately, I found disc to be the not viable spec of the 3, at least not until you can get into enough pvp gear that has the spirit on it ot increase your throughput/mana regen, and then immediately start working on building up haste and intl. Since heal is the new Flash Heal and it heals for 1/3 the amount and has twice the cast time you should do what you can to make it as useful as possible. Also, I found that a lot of disc priests were not using Holy Fire before their Smite spam, which is a huge mistake, as Holy Fire increases damage done by Smite by 20%. Also, if you are having huge mana problems then you may want to consider stocking up on potions and use flasks that increase spirit instead of SP. THE GOOD THING: Lots of vamp ups for patch 4.0.6 for disc priests to make it a lot better!
Holy: Holy was the other spec that I found to be overwhelmingly OP. The mana cost is not nearly a problem now, and the heals that go out especially with Greater Heal are huge. Most of my spells seemed to proc a lot, and I am not even carrying that much crit. Since you can use Chakra at ALL TIMES it should be used that way. Most people forget about it most of the time. Also, do not forget about Holy Word: Chastise, which gives you an additional chakra-like state. Still, stacking spirit is a must and after that is haste.
If you have not checked out the updated patch notes about priests here they are listed:
• The duration of Levitate has been increased to 10 minutes, up from 2.
• The mana cost of Power Word: Fortitude has been reduced by approximately 68%, making it roughly equal to the cost of Mark of the Wild.
• The mana cost of Power Word: Shield has been increased by approximately 31%, but its effect has been increased by 208%.
• The mana cost of Renew has been reduced by 24%.
• The mana cost of Shadow Protection has been reduced by approximately 65%, making it roughly equal to the cost of Mark of the Wild.
• Talent Specializations
• Discipline
i. Focused Will now procs when the priest is critically hit, in addition to its current effect.
ii. Improved Power Word: Shield now also increases the amount absorbed by an additional 15/30% when cast on oneself.
iii. Pain Suppression is no longer dispellable.
• Holy
i. Blessed Resilience will now proc when the priest is critically hit, in addition to its current effect.
ii. Chakra states now last 1 minute, up from 30 seconds.
iii. Desperate Prayer now heals the priest for 30% of their total health, up from a very subpar value.
iv. Holy Concentration now increases the amount of mana regeneration from Spirit while in combat by an additional 15/30%, down from 20/40%.
v. Holy Word: Chastise now has a 30-second cooldown, up from 25. In addition, it properly breaks from damage.
vi. Lightwell's health has been increased by 50% (for PvP purposes).
vii. Serendipity now has Spell Alert and Floating Combat Text feedback support.
viii. Surge of Light can now also proc from Flash Heal and Greater Heal, and can now also critically hit.
• Glyphs
i. Glyph of Mind Flay no longer requires Shadow Word: Pain to be on the target.
ii. Glyph of Pain Suppression has been renamed Glyph of Desperation and now allows Guardian Spirit to be cast while stunned, in addition to Pain Suppression.
iii. Glyph of Psychic Horror now reduces the cooldown of Psychic Horror by 30 seconds, down from 60.
iv. Glyph of Spirit of Redemption has been converted into Glyph of Prayer of Mending, which increases the healing done by the first charge of Prayer of Mending by 60%.
That is all for patch 4.0.6