Thanks for the comments, however sparse
I think too few people use Iouns. 95% of new books that have items don't have Iouns and its a pity. Nice, always useful. If you design, for example, a kickass headband, keep in mind that most people would still prefer a Headband of Intellect unless your headband is even stronger. Iouns, on the other hand, can always be used.
Just did some playtesting with the items, allowed three of my players to create 25 th. level characters with 2 mil. GP to spend on items. One of em has created a rather nasty army for himself (Rod of the Undead Warlord, Gar'Udok's Banner, Ring of Commanding, 3 levels in the Mechanist PrC and 7 in the Servant of Maldragar. I am speaking Fiendish Skeleton Archers with automatically enchanted bows here.... and roughly 800 of them to back up his main army of 1200 zombie warriors with Steam grafts).
I could bump the Soulstealer weapon up a notch in Market Price. Still, most people would still be in two minds about using it, due to it's cost and the fact that it doesn't give any bonuses to start out with. Most people abhore loosing levels, even though it would pay of in the end, eventually.
Did you find the Heart of Darkness to be a fair item, btw? Its also one I have been having doubts about...