Well, we all knew this post was coming. Haste has become the Armor Penetration of Cataclysm hunters, in that it's pretty much a mystery to a lot of people. The EJ thread has tables with focus costs and abbreviations for things like "left GCDs" and "used GCDs" and it's different for different specs, different tier set bonuses, and so on. Further, even for those hunters that go to FemaleDwarf or use SimCraft, it remains confusing. A hunter can run her gear through FD and see that the site is saying haste is worth a lot of DPS for her. So then she runs a new sim having reforged everything to haste and suddenly haste isn't worth very much. What in the world is going on when this happens?
Well, what's happening is that she's running into these haste "plateaus". Basically, the value of haste is increasing rapidly as she gets closer to something big such as "being able to use another Steady Shot in between every single Chimera Shot". Once she's got that extra Steady Shot wedged in the middle of those Chimeras, however, she's suddenly thousands of haste rating away from being able to squeeze yet another in, so haste's value drops precipitately. This also holds true for Survival hunters who could, for example, theoretically reach a point where they could fire 4 cobra shots in between every Explosive shot without pushing that Explosive back.
Between the tables, racials, haste rating, different specs, and conflicting sources, it can be difficult to tell what you should be gearing for, so the goal of this post is (as always) to make it easy. I think the best way to do that is to start by addressing the answers that are pretty much always good answers.
Good Answer #1: Marksmanship and the Five Steady Shot Cycle
Marksmanship is the top raiding spec as of the writing of this post, and it turns out that there's a simple rule for haste that will yield good results for Marksmanship hunters of any gear level, working on any content. Here it is:
Get at least 13.5% character-sheet haste with a spec that includes 3/3 Pathing and doesn't glyph Chimera Shot.
That's it. For real. Open up your character sheet, expand out the stats, and hover your mouse over the "haste" section under ranged. This automatically includes buffs, talents, racials, and gear, so make sure you're not picking up random extra haste from a party member or anything.
If you're curious, the theorycrafted minimum is 12.97%, but that would be with perfect play and 0 latency, two things I certainly don't have. You might need a little bit more or a little bit less - the way to test is to go to a target dummy partied with someone that supplies the 10% haste buff. You should be able to maintain the following cycle indefinitely:
Chimera Shot -> Steady Shot x2 -> Arcane Shot x2 -> Steady Shot x3 -> Chimera Shot
When MMM gives you a free, instant Aimed Shot, you can place it either immediately after the Chimera Shot or immediately after the Arcane Shot pair, and then remove one steady shot from the trio at the end of the cycle. If you find that you're consistently pushing back your Chimera shot, get a little more haste. For reference, my latency is usually between 40 and 70 ms, and I stick with 13.5% to 14% character-sheet haste.
This is about as forgiving and flexible as MM DPS gets, and isn't excessively punished by movement or target switches. It's easily altered to account for procs such as the instant Aimed Shot and the Tier 12 4-piece bonus proccing. It does very competitive damage, and depending on encounter, gear, raid buffs, and debuffs on the boss anyone can use this style of play to achieve results in the 20-30k single-target DPS range.
Good Answer #2: Survival and the Three Cobra Shot per Explosive Shot Cycle
Just as Marksmanship DPS is constrained by the twin requirements of the Chimera Shot cooldown and the duration of the Improved Steady Shot buff, Survival is constrained by the Explosive Shot cooldown. At current gear levels, there's exactly one reachable haste plateau for Survival hunters:
Get at least 20% character-sheet haste with a spec that includes 3/3 Pathing.
The theoretical minimum haste rating for this is 757, but again, I'd recommend going a little over. You want to be able to do this:
Explosive Shot -> Cobra Shot x3 -> Explosive Shot
Good Answer #3: These Aren't the Only Answers
There's actually a fair amount of flexibility with regards to haste and haste plateaus. There's at least one fairly progressed hunter who has over 15% haste from rating alone, glyphs Chimera Shot, and hardcasts Aimed Shot. This playstyle works for him and his guild. And when T11 was more current, there was a different haste plateau for the 4-piece bonus that used the Chimera Shot glyph. There are also highly progressed hunters that glyph Chimera Shot instead of Rapid Fire with T12, still use Arcane Shot, and aim for yet another haste plateau.
But what is relevant for the players on the bleeding edge of progression is not necessarily relevant for the average hunter. You could absolutely do worse than follow the advice above. These are very popular playstyles, and they're popular with good reason. Maybe you'll get bitten by the theorycraft bug and decide to do your own tables and spreadsheets of DPS cycles and leave these behind - that's fine! Hooray! But until then you can raid and do great damage with either of these very simple guidelines.