Spirit Link Totem. The dev's expressed a desire to add more cooldown options for resto shamans (who were limited basically to just Nature's Swiftness, if you don't count Mana Tide Totem) and Spirit Link Totem is a surprisingly elegant solution. As with any new ability but perhaps moreso with a cooldown based ability is a learning curve for both how it works and when to use it. Hopefully I can help clarify some about both.
How it Works
There are two elements to SLT that must be addressed in order to maximize it's potential. First, damage is reduced by 10% for any party or raid member within 10 yards. That is pretty basic, but rememeber in order to benefit from the 10% less damage you must be standing within 10 yards. Also this is not just limited to the party the shaman is in, but anyone in the raid. Secondly, every second (for 6 seconds) the health of all players standing within the circle will be redistributed, so that all players have the same overall percentage of their health.
As a rough example, let's use the example of yourself and one other player standing in the area. For the sake of simplicity let's say you have 100% and the other player has 50% health. On the next tick of SLT you will both have 75% health. More on the technical side, it does this by healing the lower players and dealing nature damage to the higher players. On the PTR people were reporting the ability to resist the damage portion, it does not seem to be this way on live.
Regardless of the changes to healing in Cataclysm, there is no shortage of AOE raid damage. There are many times during a fight where a SLT would be awesome and here are some more specific examples of when it's ideal.
You can use this anytime during Magmaw, as the Magma Spit he does is random and causes splash damage. The damage caused by the adds is hard to mitigate with SLT due to the high mobility of the fight. Because the damage is not too massive on the spit, save it for when people are simply getting low.
OnMyLawn (Omnitron) Defense System
The best place to use this is probably during Magmatron's Security Measure, aka flame AOE ability. If a player is about to be hit with Flamethrower and may not survive, go ahead and use it then. Just remember that SLT has a limited range and getting near the acquired target may put you in danger yourself.
The best time to use SLT is during the green phase. The Debilitating Slime increases damage taken for everyone (raid members included). If there are a few adds loose or some new ones coming out they can quickly chop a healer or clothy in half.
Naturally the best time to use SLT is during Feud while taking Caustic Slimes. Make sure you coordinate with your other healers and what cooldowns they may be using to spread them out over the multiple Feuds you will be dealing with. As long as everyone is relatively topped off prior to the first caustic slime, wait until that first hit to drop SLT. This will maximize the duration and effect.
Due to the mechanic of the fight, this is a difficult fight to utilize SLT for. In fact, you may be better off using it instead on the trash preceding Atramedes - those guys are tough! The best time I can find to use this is right before an initial Searing Flame tick. This will happen multiple times during the fight so hold on until people are low. The frequent and high mobility of this fight make using SLT challenging at best.
There is an overabundance of random opportunities to use SLT during this encounter. As for specific mechanics there are two that help SLT really shine. If the raid is at low health a Electrocute could mean wipe, so consider SLT for this situation. If you are doing 25 man, you can get better use out of SLT during the platform phase to mitigate some Blast Nova damage.
If you get the whelps to manage, SLT can greatly reduce the strain (if it gets to be too much) on the healers during that massive flame AOE breath from the behemoth. Short of that one mechanic just be aware of potential dangerous situations to drop SLT.
Twin Dragons of Sparkly Doom (aka Theralion and Valiona)
There are two great situations in which you can utilize SLT during this fight. The Blackout's during Valiona and the Twilight Meteor during Theralion. Under normal circumstances you can heal through these so hold on to SLT until it could be a game changer.
Using SLT here is tricky. This fight involves being spread out for most of phase one and two thus the tricky aspect. If you can find a group stacked up well enough SLT is great during Ignacious's Rising Flames. By using it early it should be available during phase three as well. During this last phase it's important to also spread out to reduce lighting damage, however if the Lava Seeds go out near a group, SLT can make a quick save.
During phase 1 you can use SLT to soften a missed interrupt on Depravity. It can also be used during shadow orders if the healing begins to get overwhelming. During the last phase you can use SLT as a buffer when the corruption starts to get high to stay alive as long as possible.
Conclave of Wind
The ideal placement for SLT is during Nezir's ultimate. Provided raid members are alert for a potential frost patch, having them stack to get SLT's benefit can help out significantly.
This is another fight where lots of movement and being spread out is the norm. During phase two when the Acid Rain starts to stack up is a good time to utilize SLT. Too bad our totems don't get little wings and follow us around, right?