Effects that last "for as long as that creature has a [kind of] counter on it," such as Aven Mimeomancer's, stop applying to Me, the immortal once it leaves the battlefield
While this ability is powerful, the consistent slowness that comes with the ability draws out a given game, often making it last significantly longer than it would normally take, causing significantly fewer spells to be cast each turn
While not the best effect on rate, this works in a pinch when you need to answer a troublesome creature or planeswalker
It might be too early to call it a staple, but its definitely more than just a niche madness card
I think it falls a little short of being noteworthy as a commander, but it still pairs well with a creature like Guild Artisan or Scion of Halaster
These lists can help you determine a price , even if you might not want to sell to that particular dealer