Waist of my time (so far). For 1 it does not have 1/2 the casting time, as RamgarSK says. Casting time on ToV, 3.0 (1.6) Casting time on Drain Soul 3.2 (1.8)
2. With a mana cost of 43 points higher than Drain Soul, and only 10 more hp's, its not very mana eficient.
3. It has a longer recast time than Drain Soul. DS can be chain cast, ToV has about a 8 second recast time.
4. I am lvl 65 and have raided the PoP zones. So far I have had 0 resists on either of the spells.
IMHO skip ToV and wait for Touch of Innoruk. 500+ hp LT. and with Touch of Life AA's, its easy to get crits in the 1k.