My understanding is haste caps at 16% but have never seen that listed anywhere so might just be one of those numbers that gets thrown around

yes, haste does have a cap. it is not, however, realistically possible to hit it in current gear as it starts taking more and more haste to get less and less cast time reduction

Yes, there is a cap on haste. You can not push your GCD below one second, after that point all extra haste is wasted. Unfortunately, I don't have the numbers to say what amount of haste rating it takes to reach that cap.

The only cap on haste is the global cooldown itself: if haste would reduce the cast time of the spell to under what your global cooldown is (1sec minimum, with ~1640 haste, it looks like (Wowwiki)) than you can't cast any faster than 1 per second becuase of the GCD.

Also might be worthwhile to note that different specs value stats differently. As a pvp frost mage, for example, I value haste very highly, so I can take advantage of shatter more. On the other hand, as a pve frostfire mage, I value crit more than haste because of the insane critical damage bonus frostfire builds have.

There isnt a hard cap, no. The soft cap would be 33%, enough to get you from 1.5s GCD's to 1s GCDs...but haste still has value on other spells if they are longer then 1.5s.

Its like the 'yellow' hit cap melee has, kinda :)

@Sagath

Not 33%, 50%. You got the fraction upside-down.

X% haste doesn't "reduce your cast time by X%", it allows you to "cast X% more spells per time period". The formula for cast time is 1/(1+haste), with haste expressed as a decimal (ie, 15%=0.15).

It's probably been done this way to stop people from being able to make everything an instant cast by gearing like crazy toward a 100% haste cap. The model of haste we have in WoW has 100% haste giving you twice as many spells in a unit of time, so it's halving your cast time. It's worth noting that X% haste will cause an X% increase on your unhasted DPS, but also makes you burn through X% more mana per second. That extra mana burning on haste compared to crits (which don't cost any more than the normal spell) coupled with interesting procs on crits is the big reason why 1% haste costs so much less item budget than 1% crit.

I theorycraft for my shaman girlfriend quite a bit, and it's pretty easy to tune her haste around her rotation. Let's say she wants to fit 5 lightning bolts into the 8s cooldown of her lava burst. I take the 10s total cast time of 5 lightning bolts and divide by the 8s of the lava burst to get 1.25 so she needs 25% haste. With her current gear that's just too much to bother trying to enchant/gem for, so instead she's gone for around 18.75% and squeezing 1CL+4LiB (9.5s worth) into the 8s.

Teaspoon- Godly Post. You hit haste on the head. Haste is a tool make sure you utilize your entire mana pool.