One of the most powerful ice ever printed. Engram Flush almost always hits for 4+ credits or 1-2 cards of the corp's choice, which is enormous and consistent value for a 2-cost ice. Along with Slot Machine, it was in the elite tier of code gates which fares well against both Engolo and Amina. It was removed from standard play because its numbers are crazy for a 2-cost ice. Even if they have a Hunting Grounds, it only affects 1 Engram Flush once per turn, and even then they still reveal their hand.
EF wasn't QUITE as universally useful as Slot Machine or Gold Farmer, but it was an extremely potent import at 1 influence, assuming your enemy plays better with cards. In-faction, it pairs savagely with the 3 Obokatas that you were already running and any net damage cards you might be running. When this ice was legal, it was an almost guaranteed 3-of in any Jinteki deck with 10+ ice, and almost half of corp decks ran 2-3 copies of it during Salvaged Memories.
Tactics: deploy early, rez early. Unless you have reason to suspect otherwise, choosing Event is usually the best option (or close). Most game-winning cards are events, and they're the most likely to be held in reserve for a killer moment.