I'd still like to see where most people is getting the "Haste is always better!" from. So far, mastery is still better in terms of HPM which is 99% of the time the limiting factor.
Only HoTs get increase in healing done from haste and surprisingly the HoTs are only ~60 - 80% of our healing for each fight with extremes being as low as 50%. Above 80% I don't see them ever being. (Healing distributions are from my own logs.) This diminishes the value of haste immensely in terms of HPM.
Before you ask where do I get my numbers, they're from my own spreadsheet:
https://docs.google.com/spreadsheets...nb-7ck4t-oi6Yc. As it's something I've made, I'm not 100% certain it's accurate (it has some minor assumptions as well), so if you find any mistakes, I'm happy to be proven to be wrong.
Of course when you are behind in healing and really want to press the buttons, haste is the best stat (even better than intellect, I might add). But that's impossible to calculate so you will just have to estimate yourself how important you choose to keep haste. I personally find mana more important than panic healing, so I go mastery.