It's a slow card and will be a magnet for removal if you're exiling stuff your opponents want back, but in a dedicated +1/+1 Counter deck it's a solid way to slow down opponents, plus you can even exile your own stuff to re-use ETB triggers or protect from a future board wipe
If a card that's already been on the battlefield for a full turn becomes a copy of something else, it'll still be unaffected by summoning sickness
Its untap ability adds even more versatility, which lets you reset lands, mana dorks, or utility permanents for extra value
Dont get greedy, and potentially hold back digging deeper
Once a spell has been copied on the stack, its treated as any other spell and subject to the same weaknesses
Funny enough, "Kellan, the Kid '' is a direct reference to the notorious Billy The Kid, who was an outlaw in New Mexico and Arizona during the late 1800s