(To mill two cards, a player puts the top two cards of their library into their graveyard.) The card you put on top of your library, if any, may be one you just milled, or it can be one that was already in your graveyard
Ive used it to cast things as innocuous as Cultivate all the way up to exiling a Surge to Victory and exiling my Ephemerate while I had Aurelia, the Warleader in play (spoilers: thats a combo)
For years, it was the best budget tutor, and it still holds its own today, but as more cards get printed and the formats speed continues to increase, its flaws start to become more and more apparent
Im not sure these merit much premium either, but there may be exceptions
Yes, Saproling Cluster is a key card in 3 combo(s) in our combo database, linked from the Combos section of this page.
but since those guys are bred for war, I guess it's understandable