I Think Feast Might be Underappreciated
How much of us average in CS per game? I think it's anywhere from 6 to 9 per minute, right? Which means anywhere from 33% to 20% of our farm could be converted into restored health with the Feast Mastery.
If it's a minimum of 20% of your farm converted into health and you acquire a minimum of 200 CS a game then that means you could restore 800 health per game from Feast alone.
Now, compare that the the Double Edged mastery people prefer. If you inflict an average of 25k damage to enemy champions per game without it then you will inflict an additional 750 damage to enemy champions. Now, consider how much damage you receive from enemy champions. If 3 of your enemies inflict massive burst and each one ends up dealing even a 5k damage to you then you will end up taking an additional 225 damage from champions.
Compare the two masteries with the examples stated above. +800 effective health from Feast or -225 effective health for +750 damage from Double Edged.