The aim is to, obviously, stack the best stat (haste) but to also find a balance in the acquisition of others. Too much of one at the expense of others tends to be a DPS loss. You need to find the balance between haste, crit and versa. But generally, the best bet is to pursue crit and Versa after the first plateau ( about 20% haste). It appears there are some breakpoints for haste at 19%,24%,27% and about 31%.
If you are using the Legendary Cloak(Whisper of the Nathrezim) you should pursue haste till about 31%.