For a caster any amount of haste that lowers your mainline spam spell cast time under 1 second is a waste of haste, as you will still have to wait (aka lightning bolt for an ele shaman), as even if you shot a lightning bolt in .000000003 seconds, you will still have to wait the GCD for the next spell cast, be it an instant cast or not. There may be cases where there are certain spells off of the standard GCD and could be inched into there, but I cant think of any specifics off of the top of my head. Long Story Short, for a caster dont let your mainline spam spell get under a 1s cast time.
Melee/Hunters are a different situation. I dont know specifics on what they SHOULD do in terms of large haste pools, but with auto attacks haste can scale infinitily, thus meaning theres no haste "cap" as there is for casters. If a warrior attacks every .000000003 seconds (which would require so much fucking haste...but thats not the point), his rage would more or less stay at 100% all the time, thus meaning he can spam his little heart away, unlike the casters who get cockblocked by a 1s GCD. Now I'm not sure if wands scale with haste...but if you managed to somehow get your haste so that your wand auto attack is .000000003 seconds CD...then I suppose it could work. But until you could hack that much haste onto a character...ya no.
Also energy and focus regen are based on haste so ya...good for those classes