To simply discuss what THA is, I think it would be easier for me to relate it to the Red Mage in Final Fantasy XI. This of course would be confusing for anyone who has never played FFXI, so I will keep it simple.
After having played THA in beta and now returning to FFXIV to play it again. It has ring bells of Red Mage similarity from FFXI. The class is a mix of healer, nuker and debuff - warlock type class. Seeing how many posts have talked about INT/MND/PIE, I feel that it is true at the moment to basically ignore INT everytime you get points to distribute, rather I would main focus it on VIT and then give the left overs to MND and PIE e.g. when I get 6 points I would put 4 into VIT and 1 to MND and 1 to PIE. The reason for this would be because I solo a lot at this point, and having more magic resistance and magic accuracy would not do me better good than having more HP.
Now that I've watched and played a friend's account (My own character's rank is low), it is obvious that DoTs are having a more clear effect than they were back in XI, and so in long fights I would always just DoT things up and then put in some nukes, but always keeping DoTs on the tick.
People of course have different views about the usefulness/uselessness of this class, especially when nerfs would most likely come in effect along the way. However, this would be rather the same for RDMs in XI (if I remembered correctly XI didn't make much class buffs/nerfs). But the result of an end game Red Mage + Ninja mix made it probably the best and most wanted class in all kinds of parties and raids, and of course, for any one who has seen the Japanese Taru RDM soloing Burning Circles and HNMs, they would know how beefy this kind of class would be.
I can't say the same would be for THAs but I would think that there would definily be a lot of use for it when more end game contents are released.