The item was 100% gold efficient when the champion has 1204 maximum health.
All champions have between 1260-1764 base health at level 13, therefore, this item was cost efficient for every champion in the game by level 13.
Atma's Impaler adds directly to attack damage. It was not an "on-hit" effect.
The unique passive grants the same amount of attack damage as a B. F. Sword (50) at 3333 health.
Champions with large health pools or health-gaining abilities, like Sion and Cho'Gath, profit greatly from Atma's Impaler.
Atma's Impaler was one of the most cost-effective items in game, surpassing Pickaxe on any level 18 AD champion (without HP items).
Along with being built from a gold-gen item (Avarice Blade), the cost-effectiveness was increased.
Atma's Impaler can be combined with items such as Frozen Mallet and Warmog's Armor to utilize both damage and survivability. These builds are known in the community as "Fratma's" and "Atmog's", respectively, "Fratmog's" for both at the same time.