According from my experience and from some peoples testing it out from the wiki here is the data on mana wall:
*The effect of Mana Wall appears to be 100% damage transfer to MP.
**Reduces damage taken by 50%. (100 Damage taken takes 50 mp away instead of 100 hp)
*** All forms of damage modifiers (PDT, Phalanx, Stoneskin, ect) are taken into account before mana wall effect's takes away mp.
*Because no HP damage is taken, this ability effectively prevents spell interruption from damage.
*If you get reduced to 0 mp because of damage taken from Mana Wall, Mana Wall wears off immediately.
*Damage will carry over to HP when MP is low enough.
*Mana Wall does not allow resting while you have a DOT on you even though you are not losing HP.
*While Mana Wall is active Sublimation's effect changes from -2 hp per tic to -1 mp per tic. (unconfirmed)
*Despite popular belief mana wall will NOT create an invincible like effect while manafont is active.
Keep in mind the unconfirmed status I put on some of these only becuase wiki had it too but i've used this skill many times and found all of these things to be true.
I say this skill is very powerful (more so then expected since 1hp = 0.5 mp). With this Black mages will live much more often without too much of a hindrance to there performance as well as have a new form of an anti interruption.
...but thats just what I think what is everyone elses opinion on this?
*edit added new data to the op.