Edit: Lol to the above. Started typing this post before that one went up =p.
I've had a lot of PMs the past couple of days asking about the haste soft cap, which I guess I made the mistake of throwing in somewhere in the OP last time I editted. There really isn't a haste soft cap as there is the hit soft cap, or (for tanks) the expertise soft cap. It's a much fuzzier area, which apparently confused some people. So, I figured I would just toss some numbers out there so people understand what it means and exactly why haste is so much more valuable now than it was previously or as it is for other melee classes:
To begin with, some numbers to keep in mind:
Your base rotation is 7 GCDs (10.5 seconds with 0 haste) per rune refresh (10 seconds).
Of those 7 GCDs, 3 of those are spell GCDs (in a 2H rotation. In a DW one, it's about 4).
The spell GCD can be reduced by haste, the melee GCD cannot.
It takes 32.79 haste rating for 1% spell haste.
X% haste does make you cast X% faster (or reduce the spell gcd by X%). It instead allows you to cast X% more spells than you would without that haste in a specific time period. The formula is essentially:
New Casting Time = (Base Casting Time)/(1 + (% Spell Haste / 100)).
You receive a total of 8% spell haste in a raid setting. 3% is from a moonkin or ret paladin and 5% is from a non-enhancement shaman.
So to begin with, you want to knock 0.5 seconds off your rotation, as otherwise you're sitting on runes. Doing so essentially gives you a straight up 5% increase to yellow dps (on top of the normal autoattack/pet increases). Since 3 of those 7 GCDs are spell gcds, that's 4.5 seconds worth of gcds which you need to reduce to 4.0 seconds - which means you are reducing each of those 3 gcds to 1.333 seconds each. Plugging that in to the already stated formula, that would require 12.5% spell haste. Since you are receiving 8% from raid buffs, that's 4.5% you want from gear - which is 147.56 haste rating.
That makes 147.56 the initial soft cap. The most important. It's one everyone should reach, and it's pretty much a joke to do so.
However, the spell GCD reduction aspect of haste continues to benefit you.
First, there's Blood Tap + Bone Shield. As already shown a couple pages ago, it's better to Blood Tap + Bone Shield in place of a Blood Strike than to push your whole rotation off by 1.5 seconds. However, if you can reduce your rotation by a further 1.5 seconds per minute, that essentially gives you a free Blood Strike per minute. Per minute, you have a total of 42 GCDs (18 spell GCDs) but you want to be able to do 43 for the extra Blood Strike. 43 x 1.5 = 64.5 seconds. You want to knock off 4.5 seconds from those 18 spell GCDs, which means you need to reduce the spell GCD from 1.5 (27 seconds) to 1.25 (22.5 seconds). Plugging that in to the formula once again, that takes you 20% spell haste, or 12% post raid buffs. Which requires 393.48 haste, a much harder to reach number, but something you surpass in BiS.
Then, there's Revitalize. How much you get from this can vary a ton. It depends on the fight, on the number of Resto druids your raid has, and on simply RNG. Let's assume you get enough RP for one Death Coil a minute (which would mean you have a single Rejuv up on you for at least ~80% of the fight or WG up on you for ~27% of the fight. That's high for some fights, low for others. It makes math simple, however, so going with it.). That's one Death Coil in addition to being able to Blood Tap + Bone Shield on an empty GCD, which itself was on top of cutting off that initial .5 seconds per 10.5. So, you want to do 44 gcds per minute, 19 of which are going to be spell gcds. That means you need to get your spell GCD down to 1.184 seconds. Which means you need 26.67% spell haste. Which means you need 18.67% spell haste from gear alone. Which means you need 612.189 haste, a number you aren't going to reach in any normal gear.
Then there's AMS, something you can use at least once a minute for a full bar of RP on almost every single CC fight. AMS is probably 2 DCs a minute, essentially.
Then there's HoW, something you can always spend empty GCDs on if there's nothing better.
Doing the math for those above two would be redundant - hopefully you get the point by now.
The bottom line is this: You can always use free GCDs. Always, always, always. You can, I guarantee you. Unless you raid with zero resto druids, zero disc priests, and never use AMS (in which case you have other things to work out).
So the reason I threw out those numbers (which I could have continued with, but figured you got the point) was to show that haste takes more and more to get extra GCDs out of it. For a caster, this isn't the case, since all of their GCDs are spell gcds, but since we're half and half (ish), it is. As such, while you may never hit the haste hard cap (which would be a GCD of 1 second, which requires 50% spell haste, or 42% from gear - 1377.18 haste rating), haste does somewhat decrease in value the higher you go - but until you hit that hard cap (which you won't, short of during heroism or during double procs - something like potion of speed + dark matter or potion of speed + the engineering enchant) haste is still incredibly valuable - more than it ever was pre-3.2.2 (since we never before have had such a high portion of our GCDs be spell GCDs). On top of the gcd reduction, it's still increasing your autoattack/ghoul damage the entire time, too.
TLDR: Haste is good. There's a soft cap, but it depends entirely on the specific fight, your specific raid composition, and other such matters. You'll probably never reach the soft cap, however, as at the very least it consists of the haste it takes you to bring your rotation down to 10 seconds + the haste to add a Blood Strike per minute + the haste it takes to free GCDs for DCs from Revitalize + the haste to free GCDs for DCs from AMS + the haste to free GCDs for HoW + the haste to free GCDs for the DCs from the HoWs. You won't reach it. The soft cap is probably the same as the hard cap for most people on most fights - 1377.18 haste rating. I and others probably used the term soft cap incorrectly, which led to some of the confusion. Haste potions and haste procs (engineering, Dark Matter, etc) can bring you to the hard cap, which is why they're not quite as valuable as the sim let's on. Even if they don't bring you to the hard cap, the closer you get to it, the less valuable it gets (although it never drops to the point of undesirability). It's kinda like crit - the more of it, the less valuable it is. But until you hit the hard cap (crit has one, btw), it never changes all that much in value. Not like ArP, which is the opposite (increases the more you have) but at an exponential curve.