One of the best opening $4’s. There’s very few occasions where I wouldn’t open with Moneylender; it’s more useful than Baron early on because it trashes (rather than discards) and works with Coppers (which are more plentiful than Estates). Moneylender even works with Chapel: Moneylender can bootstrap Chapel decks into $5’s and Golds as quickly than a Silver while thinning the deck faster, and if there are no worthwhile $2’s, then drawing Silver/Chapel is just as useless as drawing Moneylender/Chapel.
Unfortunately, there’s not much use for Moneylender outside the early game. Look for a way to get rid of it, even before it trashes all 7 Coppers; you’re just not as likely to draw it with your Coppers after three or four trashings. In the midgame, if you can draw your entire deck, then Moneylender can be a decent defense against Mountebank and Ambassador. And if you feel especially daring, it might be worthwhile to spend some extra Buys on Coppers if you have excess card-drawing capacity. More often, though, the Moneylender just ends up getting fed to the Salvager, Forge, or even the Chapel.