When I get it on Eve, I get it right after my jungle item, since it's only 350. You can stack it with ganks/fights and passively. The extra damage can really help. Plus if you snowball, after the upgrade, you can have like 1-2k gold on it, since you don't really sell it till you're on your last item (but what game goes that long anyway?). Basically, gold gen, plus bandit. It also doesn't bother those like Eve or Elise who struggle to clear waves anyway, so the passive getting locked on minion kill doesn't hurt them.
I upgrade to frostfang right after getting runic echoes. There's no downside to this item since it basically pays for itself. And again, being sticky with the tribute passive helps a lot.
For Eve specifically, it synergizes with her really well because of her Q (3 strikes. 3 stacks on spelltheifs before going on cooldown).
Edit: I also forgot about the CDR Frost fang gives. Cheap CDR on a fairly efficient item.