I'll just give my opinion
on this matter from having healed all of tier 14 as a Mistweaver Monk and currently having decided to return to my Discipline Priest after the 5.2 changes to Mistweavers.
Currently, as Fistweaving is currently implemented, I find several issues with it.
It requires far too much commitment to be effective. A Mistweaver must Fistweave or Mistweave; you cannot do both and maintain mana and effectiveness.It is impossible to intersperse a ranged healing style with Fistweaving, something Discipline can do with Atonement easily.The cost of a mistake such as having two stacks of Serpent's Zeal drop or jabbing twice in a row are far too cost prohibitive, and mistakes that are easy to make.Monitoring Mana Tea stacks is extremelly difficult without an addon such as Weak Auras.Fistweaving has a range that is too small.
These issues are ones that Discipline priests do not have, I can easily intersperse my Atonement healing throughout my normal priority system and not see myself run out of mana or be negativley affected by seeing my HPS drop. Add to that the fact that I have Archangel available that will buff not just my Atonement healing but all of my healing for a significant amount, and that the buff can be appliadle as a mitigation style healing over HPS (SPirit Shell) and I cannot see how someone cannot view Atonement as being better than Fistweaving.
The big issue that makes Atonement greater than Fistweaving for me is the fact that Archangel is available through Atonement healing, and that it provides a static 15% healing buff across the board, the fact that I do not skirt disaster by using Smite twice in a row by accident, and its excellent mana consumption to heal ratio.
Don't get me wrong, I really like my monk and I had wanted to continue healing on him for the foreseeable future. However, with the current implementation of Mistweavers we have a clunky and difficult to wield healing spec that is not intuitive and doesn't really work well with the existing healing classes. It is far too punishing for simple mistakes, and doesn't bring enough secondary utility spells such as mana regeneration and damage mitigation. Revival is also a fairly weak raid healing cooldown when compared to other existing cooldowns such as Tranquility, Power Word: Barrier, and Divine Hymn this is mostly evident in 25-man settings though.
On fights that do not have a damage modifier effect Discipline will outperform Mistweavers 95% of the time, it's a difficult reality for those of us who enjoy monk healing. The fact that monks NEED
a damage modifier fight to maintain competitiveness against classes such as Holy Paladins should demonstrate their overall ineffectiveness with their current implementation. I have not given up on the class, however, I remember the early days of Death Knights and I know that in time the class will have its kinks ironed out and will come into its own as a respectable healer; however, as things currently stand they seem to be more of a liability for progress than a catalyst for it.