Based on the evidence I've seen, and reading on other forums, it looks like the spirit change will be pretty good to priests in general. Currently, 1 spirit is worth 0.625 MP5 OFSR. It looks like the current estimate for MP5 OFSR in 2.4 will look like this:
MP5 OFSR = Spirit * (Intellect)^0.5 / 21.45
In general, if you have less than ~180 Intellect, you'll actually see it as a nerf. But, since any level 70 priest with less than 180 Intellect is SERIOUSLY lacking in gear, most of us will see this as a buff. In fact, if you have around ~460 Intellect (which is probably not even full Kara gear), you'll see about 1 MP5 OFSR. At mostly T5 gear (say, 550 Intellect), you'll see just under 1.09 MP5 OFSR, and at the T6 level (I'm guessing about 650 Intellect) you'll see around 1.19 MP5 OFSR.
IOW, for those of you having mana trouble and not very good with playing with the FSR, you'll see a noticable improvement in your mana regen. For those of you who have mastered the FSR, you'll probably NEVER have to use a mana pot if you have a Shadow Priest, and possibly can speak with your raid leaders about actually putting the Shadow Priest with some DPSers so he can get buffs to up his DPS AND up the over-all raid DPS.
This also means that the tradition of using +9 Healing / +2 MP5 gems is pretty much out the window. Assuming T5 has 550 Intellect Buffed (probably considerably higher, but I'll play conservatively), and 80% IFSR (which is, again, conservative), a +9 Healing / +4 Spirit is worth +10.4 Healing / +1.92 MP5. IOW, with those conservative numbers, it's better (surely, +1.4 healing > +0.08 MP5 by virtually anyone's standards, unless you value 1 MP5 18 Healing or more), and if you get an Innervate, have more Intellect, or spend less time IFSR, it get's that much better.
This also means, of course, that +8 Spirit (worth +2.8 Healing / +3.85 MP5) completely destroys a +3 MP5 Blue gem. And, it means that the +9 Healing / +4 Intellect option for hitting Yellow sockets is more likely to be worth it. At those numbers, assuming Kings, makes each spirit worth + 0.004 MP5 OFSR. Assuming 550 Spirit buffed at T5 (again, conservative), you'll see the average MP5 value go from 264.59 MP5 to 265.64. IOW, it'll be like socketting +9 Healing / +4 Intellect / +1.05 MP5, and thus you'd only be trading -0.85 MP5 for 66 Mana (assuming Kings) and whatever the socket bonus is.
Of course, the MP5 effect grows at the square root of Intellect, so that will be worth slightly less as your intellect grows, but may make it a comparatively powerful gem for Kara or lower geared players, particularly where the initial mana pool is an issue.
All in all, this looks like a MASSIVE buff to level 70 Holy Priests (and, likely, druid), and I'll have to spend a lot of money resocketting much of my gear.
MP5 OFSR = Spirit * (Intellect)^0.5 / 21.45
In general, if you have less than ~180 Intellect, you'll actually see it as a nerf. But, since any level 70 priest with less than 180 Intellect is SERIOUSLY lacking in gear, most of us will see this as a buff. In fact, if you have around ~460 Intellect (which is probably not even full Kara gear), you'll see about 1 MP5 OFSR. At mostly T5 gear (say, 550 Intellect), you'll see just under 1.09 MP5 OFSR, and at the T6 level (I'm guessing about 650 Intellect) you'll see around 1.19 MP5 OFSR.
IOW, for those of you having mana trouble and not very good with playing with the FSR, you'll see a noticable improvement in your mana regen. For those of you who have mastered the FSR, you'll probably NEVER have to use a mana pot if you have a Shadow Priest, and possibly can speak with your raid leaders about actually putting the Shadow Priest with some DPSers so he can get buffs to up his DPS AND up the over-all raid DPS.
This also means that the tradition of using +9 Healing / +2 MP5 gems is pretty much out the window. Assuming T5 has 550 Intellect Buffed (probably considerably higher, but I'll play conservatively), and 80% IFSR (which is, again, conservative), a +9 Healing / +4 Spirit is worth +10.4 Healing / +1.92 MP5. IOW, with those conservative numbers, it's better (surely, +1.4 healing > +0.08 MP5 by virtually anyone's standards, unless you value 1 MP5 18 Healing or more), and if you get an Innervate, have more Intellect, or spend less time IFSR, it get's that much better.
This also means, of course, that +8 Spirit (worth +2.8 Healing / +3.85 MP5) completely destroys a +3 MP5 Blue gem. And, it means that the +9 Healing / +4 Intellect option for hitting Yellow sockets is more likely to be worth it. At those numbers, assuming Kings, makes each spirit worth + 0.004 MP5 OFSR. Assuming 550 Spirit buffed at T5 (again, conservative), you'll see the average MP5 value go from 264.59 MP5 to 265.64. IOW, it'll be like socketting +9 Healing / +4 Intellect / +1.05 MP5, and thus you'd only be trading -0.85 MP5 for 66 Mana (assuming Kings) and whatever the socket bonus is.
Of course, the MP5 effect grows at the square root of Intellect, so that will be worth slightly less as your intellect grows, but may make it a comparatively powerful gem for Kara or lower geared players, particularly where the initial mana pool is an issue.
All in all, this looks like a MASSIVE buff to level 70 Holy Priests (and, likely, druid), and I'll have to spend a lot of money resocketting much of my gear.