Shadow Dance, how I wish thee were how I once knew you. This talent was drastically rebalanced from essentially 4 consecutive Vanishes to 4 consecutive openers. While any and every Rogue can see that 4 Vanishes was the more powerful version, it looks like we're stuck with the current opener flavor. In addition, since the Rogue will no longer Vanish, your target will almost ALWAYS be in combat (any smart target will put themselves in combat when fighting a Rogue), so I would write off the ability to Sap completely. It does however add the ability to use the recently buffed Premeditation during the Dance, which is another buff to the previously neglected 31 pt Sub talent.
With Ambush being 45 energy, Cheap Shot being 40 energy, and Garrote costing 30 energy (assuming you picked up Dirty Deeds and Slaughter From The Shadows) using the 4-5 possible openers would cost a ton of energy. If somehow you had 5 targets to Garrote all within 2 seconds of each other, you would need 50 energy going into the Shadow Dance and you would leave it with 0 energy. If you wanted to Ambush 5 times, it would cost 225 energy, making it literally impossible to pull off considering Rogues can only have 120 energy at a maximum and with the removal of energy ticks it would be impossible to "time", as you will only gain 100 energy during the duration of the Shadow Dance.
The reason that most Rogues give me when I ask them about Shadow Dance is easily summed up: "5 Improved Ambushes bro!" With a fairly geared Rogue capable of getting Ambush to a nearly 90% critical strike chance, and the ability to use Ambush 3 times in 3 seconds (the first before Shadow Dance, then two immediately after), that could add up to some serious damage. I have seen my own Ambushes crit for well over 3k against cloth-wearing targets. 3 Ambushes in a row could put over 9,000 damage into a Mage!
Attempting to use the full 10 second Shadow Dance duration is very difficult. A 5 Ambush combo over 10 seconds (hard to do considering that targets move, though Waylay helps slightly) that requires you to start with full energy and leave you completely energy bankrupt doesn't sound like a great use of energy or time to me.
It can be useful in PvE, where enemies typically don't team up on the player. In PvP however, during the Shadow Dance duration there is plenty of time to heal up your target, which makes the extended burst capability almost nonexistent. The 2 second cooldown on abilities means all healers could cast a quick heal in between ambushes, or even CC you for the duration. In addition, if you try to spread your Shadow Dance across multiple targets, you will be losing your combo points on the previous targets. I did discover that it basically negates the Hunter's spell Flare and the Druid's Faerie Fire which would previously stop us from even Vanishing, so it does help us against those classes.
The only possible viability of all 5 openers that I see is to either: A) Cheap Shot an entire group, though not all at the same time. If you caught a group of 3-4 players or mobs clumped together, you could Cheap Shot each of them, providing a huge amount of CC in a very short time, allowing your team to coordinate cooldowns to shift momentum in a fight very quickly. If each of the Cheap Shots lasted full duration, it would be 20 seconds of stuns applied in just 10 seconds. However, you are then left completely out of energy in the center of the enemy group: not a place that a Rogue should find themselves.
Option B would be a very controlled stunlock against a single target. However, without the Vanish component, you are very vulnerable to being crowd controlled for the duration of your Shadow Dance. Using Shadow Dance can extend a stunlock (especially against casters) for quite some time, and is one of the key draws of the talent. The ability to use Garrote to silence is very effective since Cheap Shot will be on diminishing returns very quickly if used more than once.
I will say that in its current form, I am very disappointed with Blizzard's attempt at a 51 pt Subtlety talent. The previous iteration provided extreme defensive and offensive options, and really would've added some finesse to the deep Sub Rogue's lineup. It was a very classy talent, and now it is simply reduced to an Ambush spam for big crits and instant gibs. Against highly resilient targets with healers to back them up, I don't forsee the return of the pre-TBC Ambush Rogue two-shotting people outside of BRD. Blizzard has taken such great strides to avoid that style of gameplay, and it is time for Rogues to let go of the dream of returning to our glory days of being completely unstoppable.
Dueling outside of Orgrimmar tonight, everyone trying out their new talents, taught me that true balance is entering a fight without assurance of victory. I have dueled countless Gladiator-quality Warriors, and it was nearly impossible to defeat one as Shadowstep. They can turtle with their shield up and I get trashed every time. But now, with Dismantle, I actually have a fighting chance of taking down a Tauren in plate. I entered every duel with the knowledge that I could easily be defeated: I, with my new spec; them, with their new spec. It was Russian roulette in Durotar, and it felt good to know that I could die at any time. I feel the true victory is adapting, reacting, and ultimately surpassing your opponent in a give-and-take of cooldowns and abilities; not catching a Mage drinking and Ambushing 3 times while they curse themselves for wearing PvE gear outside of town.