Warwick is the only hero where Wits End can truly shine..

I think it's best on Twitch, Teemo, and possibly Trist.
You want people who has good attack speed. Twitch fits the bill best in my opinion because Spray and Pray means he can drain an entire group in seconds. Teemo is already all about debuffing, so he's good. Trist has good range, and can start draining people early.

Most people think Wit's End is anti-mage. I disagree.
I think it works best on tanks.
They often don't stack nearly as much mana. With good harassing, the tank will be out of mana even before entering the fight. After the tank is gone, the entire team will suffer.

I agreed on the Low Mana Pool champ counter. I play Udyr and when rapid fire ranged characters grab it, they put me in a very bad spot. No stance dancing means I can't throw up my damage shield or turn up some phoenix heat, just stuck stunning every 6 seconds on an auto attack.

Wit's End on Warwick needs no mention.

In particular, it works well on ranged heroes that have opportunities to poke. It's situational, but extremely useful vs. heroes with low mana pools, or heroes that run in extremely delicate mana balances; I just played a Tristana game where I rushed Wit's End as my first item because I was soloing vs. Heimerdinger middle.

Any time he ran in to Rocket or Grenade, I just sat through it and auto attack burned more of his mana than he used to cast the spell to begin with. Whenever he was low mana, I made it a priority to hit him occasionally just to keep his mana pool too low to cast any spells. It's doubly useful against casters in that it has magic resistance not only in the item, but the build up as well.

Edit: That game also had an Alistar. Any time he would pop his ulti, I'd run back to the outskirts, pop Rapid Fire, and white hit him all day. He'd run out of mana quickly and be completely unable to Heal after the duration, affecting his entire team as a result.

There was only one time I found this item game changing, and a twitch with a fair bit of attack speed used it on my rammus. Rammus has high cost spells and a low mana pool.
In my opinion it is best for disabling tanks.
And second best in disabling carries with mana. (Nidalee and Ashe and Trist ex).

As they have low mana.

For against mages it isn't so good. As the minus to their huge mana pool won't matter much. In a team fight they blast their four spells and are done for a while. You won't blast enough off in most cases to make a diff. Where as rammus might have enough to go through his spells once. And so anything you do to it will make him gimped. I am quite glad no one uses it against my nassus.

I've never tried it, but a buddy of mine rushs it on twitch, and he has a few 30 kill games, so it seems to work well, but I usually get it on Warwick since its the only way to really improve his ult.

Wits end is ok if you have a character that can hit the AS cap. The mana drain actually hurts melee characters who rely on mana more so then "casters".
If they are low mana to begin with, it can completely cripple characters like Poppy & Ashe who oom easily.

With that said, as much as I like the item there always seems to be something else to buy which is better.