You've all heard it.
Someone opens a booster pack of Invasion and yells:
"No!!!! Not a Leech! They all SUCK!!!! Why me?????"
No, they don't all "suck". Let's start with the Black one, shall we?
Black spells you play cost B more to play.
B: ~this~ gets +1/+1 until end of turn.
For 3 Mana, you get a Pumpable 2/2. That is incredible. You can Ritual it out on turn 1, then on turn 2 attack, and make it a 4/4.
I know, what about the drawback. It isn't one. With an early Leech on the table, your opponent has to deal with it quickly. You can hold back on your Black spells until the Leech is destroyed, then play them as normal. In a straight black deck, that leech just gets bigger every turn. If it's still attacking by turn 4, you own the game.
Next, I'll tackle Alabaster Leech.