I used the Heartharena tool a couple times, it's certainly helpful when you first go into arena don't know anything, and I like that it keeps track of synergies (somewhat). After you've got your hand in a few arenas though, it's not worth the time to enter in the cards. It also values card rankings over the curve immensely despite saying it dosen't. We don't want too many 5 cost cards...but keep pick the 1-point higher ranked 5 drop!
As I'm still newbish at arena, I just ctrl+f on their
tier list when I come to a decision where I have no clue what to value the cards at. Much faster to use, and lets me put my own thought into drafting.