Depends on your spec to begin with. Either way, it's still not possible to sim to a 100% certainty I think.
For Demo, it was previously said that 35% was the "cap" to go for. But with how stupid good haste is I'm thinking it's probably our most valuable secondary even after that point. No idea about the other specs.
Edit: Did an Askmrrobot Gearing Strategy simulation and it gives these values:
I currently have 43% haste and I must say shit is getting funny at that rating, DPS is certainly not lacking besides those retarded mass AoE burst down in 3 seconds pulls which are dominated by melee.