With all the new meta gems that are out, you may be asking yourself which gems should I use for which situation. This is especially true of druids since there are so many roles they can take. Let me go through different roles and tell you what I feel is the best meta for each.
Feral Bear Tank - If you are a feral druid tank, your meta gem did not change in the patch. You will still be using the Austere Shadowspirit Diamond as your preferred meta gem. Since your tank set should be different from a feral kitty DPS set, there should be no sharing options due to the increased armor percentage.
Feral Kitty DPS - You get a new meta gem with 4.0.6, which is the Agile Shadowspirit Diamond, which gives Agility and a Critical Damage Increase. Unfotunately, it may not have dropped yet for anybody on your server. If that is the case, you can go with the Relentless Earthsiege Diamond until the new one drops.
PvP Feral Druid - This is gonna be a tough call and usually will be personal choice, but I would stick with the metas you use for PvE for PvP. The point of feral bears in PvP is to be unkillable so you will need more armor and health. For PvP kitties, you will want to do as much damage as quickly as possible.
PvE Moonkin without a Healing Offspec - The one you will ultimately want is the Burning Shadowspirit Diamond, which is Intellect plus Critical Strike Damage. Until then, you will want to use the newly redesigned Chaotic Shadowspirit Diamond.
PvE Resto without a Balance Offspec - The one thing with restoration druids is that if you don't run out of mana, you can heal people. That is why the preferred meta gem for restoration druids is the Ember Shadowspirit Diamond. (Intellect + 2% More Mana)
PvE Moonkin/Resto with a Healing/Balance Offspec But Different Helms - If you are not using the same helm for both balance and restoration, then you just follow the normal PvE guidelines for moonkins and resto druids.
PvE Moonkin/Resto with a Healing/Balance Offspec and Shared Helm - If you are using the same helm for both healing and balance, you will want the meta gem with the most bang for the buck. In this case, I prefer the Ember Shadowspirit Diamond. The Ember Shadowspirit Diamond. This will give you what you need in both specs.
PvP Caster (Balance or Resto) - This is probably the toughest call of all of them. A lot of them have things that can help in PvP, between reduced snares to reduced silences. As a caster, I chose to go with the one thing I cannot shapeshift out of, silences. I went with the Forlorn Shadowspirit Diamond, which is spellpower plus reduces silence duration.
Hopefully, this guide will help you in deciding what meta gems you would like. You are probably wondering why I covered restoration druids when I have never covered restoration druids on this blog. I just recently have changed my offspec from feral tank to restoration druid. Since I have a set of gear for moonkin, I thought it would be great to try out doing some restoration healing. I really only had to replace a ring and a trinket with hit on it. The most I can say so far is that I'm learning. Not perfect by any means, but learning.