This is what I was talking about in my Phyrexian Debaser review all that single hour ago. Instant-speed sack outlets are always awesome because they can be easily abused. Add "creature removal" on top of that and it becomes amazing. For five mana, this card rocks and is usually going to register fairly high on the "things that need to be dealt with" list when it hits the table. It does suck that it has to tap in order to sack itself for the -4/-4, but if you really need to get rid of it that badly, just sack it to its second ability. This is an all-star in the world of black Magic creatures and it is a well-deserved recognition.
Pros: Instant-speed sack outlet, -4/-4 until end of turn
Cons: Requires tap for -4/-4, requires sack for all abilities
NOTE: If you're wondering why I didn't do all four Carriers in one article, it's because it's a vertical set, not a horizontal set. Call it personal neurosis, but I don't really like vertical sets as much as horizontal ones, so I tend not to lump them together. Don't really know why. Maybe it's because I have huge 3-inch binder full of horizontal sets...