Life Tap Rank 8InstantConverts [1490 + SPI * 3] health into [1490 + SPI * 3] mana. Spirit increases quantity of health converted.
Life Tap is the core spell in a warlock's arsenal. I'd even go as far as saying that Life Tap is the spell that defines all that warlocks are about. So when I saw a mage say 'oom' four minutes into a boss fight the other day, I couldn't help let loose a little grin.
A new warlocks gets this spell at level six, which doesn't take too long to get to. After this, you should aim to get to level fourteen as fast as possible so you could get Drain Life. What this spell does is obvious from its name. In conjunction with Life Tap, leveling as affliction becomes a real breeze. Sacrificing some health for mana, and then getting that health back from anything you kill is what makes a warlock so awesome while leveling. You never need to stop and drink. You're an unstoppable force in those early levels, and just wait till you get Siphon Life.
From those early stages all the way to your first raid, Life Tap's gonna be on your bars. With my somewhat decent raiding gear, Life Tap gives me 2498 mana at the cost of that same amount of health. Something mages could only dream about. For a raiding warlock, it gets even better. Through Glyph of Life Tap, 20% of your spirit gets converted to spellpower which for myself means a 67 spellpower boost. Mind that that's completely unbuffed, the number goes higher with spirit buffs and kings!
One more great application for Life Tap, off the top of my head, is to kill yourself. While the spell on its own can't kill you, it fizzles out when you're too low on health to tap, you can use it with Hellfire to get some warlock dying action. That's how awesome we are, we can kill ourselves. Yes, Paladins can do it too but they need someone else to help them do it, so they suck anyway.
You may be wondering why a warlock would want to do this. Back in the day I used it to get past the Dire Maul entrance without needing a key for my doomguard quest, dying next to the door and spirit walking through to the other side. The same could be done for Scholomance. A more modern use is to do it when your raidleader calls for a wipe during a boss. Killing yourself gives no durability loss; so it's a neat trick to reduce repair costs.
To summarize, Life Tap is awesome, and mages suck for not having it.
No Dun Morogh critters were harmed in the making of this post. Actually that's a lie, I killed at least thirty rabbits trying to get the perfect Drain Life shot.