Pretty phat lewt wouldn't you agree...
It is quite impressive. I am not sure I would introduce that level of power to one PC and not have it carry some major long-term detriment, rather than a one-time risk. But that's me.
When I read the first line aloud to the players, it was the first time I read it myself as well.
And I think that's where your real problem is. The players now know that's the text.
For real effect, this should have been positioned like the "Head of Vecna": to use this item, someone has to shove a sword
through your fraggin' heart!!!1!! They should not have been sure this wasn't a hoax, or that they'd survive the attempt, or exactly what the powers gained should be. And it probably should have been described as a dusty, flaky, shriveled old heart-raisin that's been steeped in a slightly sticky resin. The point should have been that only a crazy and power-mad character should have been willing to try it at all - the alignment change is then kind of superfluous.
Oh, wait. Here we go. That is the text.... but it is metaphorical! It is referring to a person who has had their true love reject them, or taken from them, so that they are in desperation and despair, a broken person with nobody and nothing to lose, and the alignment change is then merely a last step of their descent to villain-hood.
Then, all the people who are merely power-mad stab themselves in the heart, but since they are the sorts incapable of feeling true love, they just bleed and die, as their hearts cannot be broken.
Thinking it was a bit strange and unpractical (isn't a dragon heart larger than the entire torso?)
Or... play it that way. The PC now has this giant semi-undead heart permanently sticking out of their chest cavity that can never really close again. Good luck finding a romantic partner that isn't crazier than you are....