I just recently had this spell made. I wasn't sure I'd get much use out of it but it was only 3k so I said what the heck. Let me say, I'm a 70 SK with 284 AA's, most of which purchased for soloing. My gear is would probably be the equivalent of high end luclin stuff.
As a level 70 soloing SK I wasn't sure I'd get much use of it as I use shroud of discord which with my AA's procs for 402 and 804 if I crit. Also the lifetap proc buff is subject to my healing AA's, the mana proc buff is not. So at this time, a lifetap proc buff is > a mana proc buff. That being said, this spell does still have a use to me.
I went on a raid recently and we took on a mob with a nasty proc. The first part of it was a 4K + DD and then it becomes a mana-draining dot. It also increased the mana cost of spells unless I'm mistaken or maybe it was cast time, (it was my first raid with this guild so I was trying to pay attention).
Anyway, I decided before we took on this mob to try this spell out. It proced enough that I was able to lifetap back my health every time I got hit with the dd portion of the mob's spell and had my mana being quickly sucked away.
I also found this useful for other non-mana draining raid mobs because it enables you to keep up theft of hate and inruku's bite on your group as well as mix in some nukes.