While he does have to use your own mana, this effect is still absurdly powerful and can deny opponents the strategic cards they need, while setting up less advantageous cards to be their next draw
One of the best targets seems like Heartseeker, which has a great effect when it equips for zero and effectively turns all of your creatures into Visara, the Dreadful
However, if a single creature with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once
It seems possible that some of the locks, like Glacial Chasm in Legacy or Phyrexian Unlife in Modern, actually end up being playable because they are pretty efficiently costed
That does not mean I wont be adding these cards to my collection though, in fact its quite the opposite
If you do, the creature enters with a number of +1/+1 counters equal to the Devour number multiplied by the number of creatures devoured
