Like Sign in Blood, Damnable Pact allows you to target a player, unlike Skeletal Scrying. There have been times where playing a Sign in Blood on an opponent has actually been a good thing to do. Given enough mana late game, you could actually draw an opponent to death with Damnable Pact.
The major issue with this particular Black draw spell is that it costs you 1BB and a life to draw just one card, and 2BB and 2 life to draw 2 cards. Sign in Blood allows you to draw 2 and lose 2 for only BB. Both of these cards are sorcery speed, as well. Skeletal Scrying is instant speed, but it hasn't seen Constructed play in ages because Sign in Blood is so much more consistent and efficient. That being said, any card that can actually kill an opponent late game is pretty good, so I find that Damnable Pact is a good blend of Sign and Scrying.
This isn't a card that you would depend on in any competitive Constructed format. Indeed, it saw little to no Standard play. It's just not a very good card in the early game. Even with Black's ability to produce a ton of mana quickly with cards like Nykthos, Shine to Nyx, this simply wasn't efficient enough to make it.
In Commander, however, life total really is the only limit to how good this card can be. Black decks in that format have ways to gain life back very quickly. Mono-Black decks also run Gray Merchant of Asphodel, which gains you life and makes your opponent lose life; that makes casting this seem a lot less deadly. But the deck that likes this card the most is Nekusar, the Mindrazer. That is a deck already focused on making people draw lots of cards and lose lots of life. So, not only can this Pact draw you cards, but it can deal double damage to an opponent. Edgar Markov is also a fan of this card.
Overall, Damnable Pact is a decent Black draw card, and one that can actually win you a game by draining your opponent of their final few life points for the cost of some worthless cards.