Granted, it is a bit expensive to cast with a converted mana cost of 5. But for what it does, Curse of Bloodletting is well worth it. It's a one-sided Furnace of Rath, and doubling all damage done to a player is pretty amazing. It doesn't include life paid as a cost (such as paying life for Phyrexian mana costs), but it still does quite a lot of work.
This card never figured into competitive Magic, but it sees plenty of play in Commander. It's one of the best Curses around.