How good is Wits End on the following champions?

Not so great on Jax and Dianna, decent or really good on the rest. WW, it's situational. Shyvana, mainly situation. Pretty much always good on Kayle. Decent on tanky teemo. Pretty sure it's a core item for Kogmaw right now.

Out of the on hit champs who could use it, there's always better items which would never justify wit's end taking up a slot

Diana: Efficient although Nashor's Tooth is the more common pickup for DPS Diana builds. For an additional resistance to burst this wouldn't hurt

Jax: Very uncommon. Other Atk Spd items are general pickups

Warwick: PreRW I've seen it picked up a few times. Post I haven't seen a sign of it. His kit naturally finds better sustain in other items

Shyvana: Never seen it picked up. I would genuinely prefer other atk spd items or Bloodrazor

Kayle: Definitely a good item and an optional pickup since Kayle depends on the atk spd. I've normally built Wits out of supportive means for ally mages

Teemo: Also a good item especially if you're going for an On-Hit Build (consisting of Frozen Mallet and Nashors).

KogMaw: Given the glass cannon that he is, of course it makes for a good item. Even back to his "machine gun" rework, he found efficiency in the item. Firsthand, I tend to build this later however.

Good on jax ww teemo at the least

Its good if both teams have a hefty amount of magic damage. You will reduce damage taken for yourself and increase damage taken by the enemy you focus.