I rarely do.
Although it's nice to send targets with low mana pools into OOM oblivion, it's much easier and safer to just kill them with Drain Life instead. I do have 3/3 Mana Feed so my demon receives 100% of the mana drained, but Life Tap does the same and much more effeciently.
At level 63 my Drain Mana drains 169 mana every second making it a total of 845 mana over 5 seconds. My Hunter, who by no means have a lot of mana, still has 3200 mana. I would need to use Drain Mana at least three times on him to suck him dry. Three times five seconds, meaning I'll spend 15 seconds doing nothing but suck his mana while he can kill me all he wants.
It's a shame it doesn't increase with spell power. And if it does, it's a shame it doesn't receive a bigger bonus.
What's your opinion on the spell and its usefulness? How often do you use it and for what?
Edited, Apr 15th 2007 9:50pm by Mazra