Yes, this is an extremely powerful effect. Being able to sacrifice a creature and draw cards equal to the sacrificed creature's power, especially in Green, is a super nifty effect. With Greater Good, you could sacrifice a creature at any time for no cost and draw cards equal to the creature's power. However, you then had to discard three cards, which is a substantial cost. But for a 4-drop enchantment (2GG) that could be used multiple times at instant speed, it's very good.
The problem with having an effect like Life's Legacy at Sorcery speed and having to sacrifice a creature as a cost to cast it, you can set yourself up for a two-for-one if it gets countered. However, if you do successfully cast this and get the effect out of it, 1G to draw three, four, or even more cards is extremely good. Even at the cost of a creature, that's definitely card advantage.
This card would've been extremely good in Standard had the very popular Thragtusk still been hanging around. Imagine playing Thragtusk for 4G, gaining 5 life, then paying 1G for Life's Legacy, drawing 5 cards and getting a 3/3 beast to replace it. Seven mana for all that value seems well worth it. Alas, that never happened, although it still could in Commander…
Life's Legacy is also similar to a card called Momentous Fall, an instant from Rise of the Eldrazi. That card also required you to sacrifice a creature, but it drew you cards in the same way and also gained you life equal to that creature's toughness. So, Life's Legacy is basically half of a Momentous Fall at Sorcery speed. But, being half the mana cost at only 2 mana, it can net you some serious advantage.
Were Life's Legacy at Instant speed like Momentous Fall, this card would be super powerful. As it is, it's a pretty cool effect. Card draw in Green is rare and precious.
Life's Legacy in Standard
There seemed to be room for Life's Legacy in the Standard metagame at the time. Several potential targets were identified early on by speculators. One popular potential target for Life's Legacy was Phytotitan, a 7/2 creature from Magic 2015 that actually comes back at the end of the turn tapped. That guy never really saw much play, unfortunately, although it was a pretty cool idea. It was also speculated that Reanimator decks at the time would sacrifice big creatures like Shadowborn Demon after using their effects to draw some cards, then bring them back later. The Demon did see some play, but Life's Legacy didn't see play in these kinds of decks.
Life's Legacy did make some competitive appearances, however. There were some Selesnya Aggro decks like this one that ran two copies of the Sorcery. These kinds of decks would use Collected Company to summon value creatures like Reverent Hunter, which could get pretty big based on your devotion to Green (how many Green mana symbols you had on permanents you controlled). Life's Legacy was a decent draw spell that could help reload your hand. It was useful tech.
Another deck that would make use of Life's Legacy was Devotion to Green, and one Green Devotion deck ran a single copy of the sorcery. It was nice tech to sacrifice a monstrous Polukranos, World Eater, a Soul of New Phyrexia, or huge Genesis Hydra. This was back when you had Nykthos, Shrine to Nyx to generate a ton of Green mana, so you could summon these guys well ahead of the curve. Being able to create massive card advantage by cashing in a big creature and drawing a ton of cards was a nice tool to have.
While it was a decent card in those decks, it wasn't absolutely necessary to run. Still, it's hard to overlook how could this card could be in refilling your hand. A big creature is great, but in Magic, and most card games, drawing cards is usually better.
While it's a good card, it just wasn't a consistent enough draw engine to see much competitive play. The good news for Life's Legacy, though, is that it did have another format for players to use.
Life's Legacy in EDH / Commander
It was clear from the get-go that plenty of Commander decks will be able to utilize Life's Legacy quite effectively. While it hasn't become a staple in any one deck, there are plenty of Commander decks that will run a copy. The most popular choice has been Omnath, Locus of Rage, in which you can sacrifice a sizeable Elemental to draw some cards and deal 3 damage to something with Omnath's ability.
Another Commander that makes use of Life's Legacy is Yasova Dragonclaw. Essentially Act of Treason on a creature, you can steal a creature, attack with it, then sacrifice it for some cards. Life's Legacy sees play in roughly half of Yasova decks, so while it's not a staple, it's a part of the strategy for many who run her as their Commander.
Xenagos, God of Revels has a lot of big creatures, and creatures that double in size with "Xena-God's" combat ability. When your hand is depleted, Life's Legacy is a nice way to get in a sizeable attack, then draw a ton of cards.
Selvala, Heart of the Wilds is a Commander who already draws a lot of cards for everyone at the table. So, why not use Life's Legacy to draw even more cards and stay ahead of the rest of the table? It's an idea many Selvala players have had, in fact.
Other Commanders that make use of Life's Legacy on a semi-regular basis include Meren of Clan Nel Toth, Omnath, Locus of Mana, Titania, Protector of Argoth, and more.
How would you play Life's Legacy?