I've been on the fence with this as well. I have a quad melded vanya robe for my blm with +26 crit in the materia slots. I am currently trying to upgrade to the Crimson Robe from CT. Reason being, it has more crit without melds, +5 int and vitality more, and slightly less accuracy that I can do without.
Now, the issue is the DET loss. I lose I think 15? Maybe 18, can't remember atm. Either way, from what I understand about DET (the theorycrafting) is it takes +7 DET to equal 1 of your class' primary stat which is INT in my case. So really, while my Vanya robe is nice and I have the extra DET there, I believe I benefit more getting the Crimson Robe. More int overall (even without to DET) plus more crit.
Even if I wanted to pentameld it, I don't think it would be worth it. Wasting all that gil on materia for a couple points extra at the cheapest route. Not enough of an upgrade for the time/gil spent. I'd rather just get the Crimson Robe and be done with it. Easier to get to.
