Hello there ladies and gentlemen.
I do think that most people use SimulationCraft and Pawn in order to find out which items work the best for their characters, but I'm almost completely sure that most people don't know how to use it properly and it makes them decrease their simDPS rather than increase it.
So, what I think they do is (WARNING: IT'S NOT THE RIGHT WAY TO DO IT):
1) Open SimCraft, enable scaling in order to find out their stat weights, simulate their character and copy the Pawn string.
2) Go to WoW, open Pawn addon, import the Pawn string so an additional line appears in items' tooltips saying whether this item is an upgrade or not.
3) Start equipping everything Pawn tells them to.
This is NOT how it works.
Stat weights are DYNAMIC VALUES and they CHANGE every time you equip a different piece of gear.
What you should be doing is STOP after you're done with step 2, you equip ONE item at a time, export your character again and simulate. Import the NEW stat weights, equip the next item, export your character and simulate. Repeat until you've tested all the slots.
You could also manually change your gems/enchants (edit the text you import into SimCraft) if your stat weights are really close to each other.
The reason behind posting is a rumour that ret paladins don't need more than 22% haste, however this is not a rule everyone should follow because the stat values are different for each character and dependant on the gear you've got. In my case, I'm currently at 31% haste which works the best according to SimDPS.
Thanks for reading.