It is an exceptionally helpful item on physical damage dealers when fighting against champions with low magic resistance.
It's also highly useful on frequent auto-attackers against champions with high magic damage, since at max stacks it gives as much magic resist as items such as the Banshee's Veil while not sacrificing a slot purely for a defensive item. Bear in mind, though, that this will be less effective against burst casters who can execute their combos before you can achieve max stacks through auto attacks.
It's also a great item on tanks that rely on attackspeed or have on hit effects, as it greatly bolsters their magic resistance, and the time it takes to steal it, doesn't matter as much, due to them being tanky anyway. Tanks that work well with this include Volibear and Shen.
Wit's End is a strong counter to Thornmail, as it not only provides the user an on-hit effect with their auto-attacks that ignores Thornmail's passive, but it also provide magic resistance that reduces the incoming magic damage from Thornmail's passive. Wit's End also increases the user's magic resistance per auto-attack and reduces the enemy's magic resistance, thus allowing the user to deal more damage with Wit's End's on-hit passive than what Thornmail can reflect back.
Considering the item's name, the 42 magic damage on hit (and the previous 42% attack speed bonus) is probably is a reference to The Hitchhiker's Guide to the Galaxy where 42 is the number that signifies "The Answer to the Ultimate Question of Life, the Universe, and Everything".