I buy this only for Udyr and XZ because i was told to... the magic damage built into it, is it strong? who should i build wits end for besides them.?

It's great for any bruiser that needs attack speed and MR. Works well on Shyvana too.

Shyvanna, Irelia and Warwick are shining examples for chars this item is awesome on.

Almost any champion that need MR and make good use of AS is a good candidate for Wits End.
Also, tanks that need single target damage and make use of AS, like Volibear.
It's a very good item, suitable on many, many champions.
It's very good even whitout the damaging passive, because MR is difficult to itemize.
Think that 42 magic damage per hit, is like having 42 extra AD, that is reduced by MR (So, more difficult to itemize for) but don't scale whit Crit.
You exchange one thing for another.

I see Shyvana, Irelia, Udyr, Warwick, Teemo, AS kayle, sometimes nasus, shen, any tank with on hit effects or that benefits from AS. Shyvana gets lower cd on her Q, Irelia gets extra true damage and becomes a stron anti mage, Udyr get bonus healing from turtle stance, Warick gets bonus healing from passive, Teemo gets bonus poison from his E, and kayle gets bonus magic damage on her E, and Shen's passive refreshes faster with AA so having a bit more MR and a more active passive is awesome.

I'm not 100% sure so don't quote me.

Wit's end is one of the more cost efficient items for anyone who can use the AS and MR, while it's very cost effecient for the amount of MAGIC damage it does.

Not so obvious is the Tier 1 items that make a Wit's End. The Recurve Bow is expensive and if you haven't built any damage yet, it is hard justify returning to base. Likewise, the null magic mantle is hard to justify to get by itself.

What I'm getting at is, it difficult to save for because you are better off buying the whole item. While it exceeds 2k gold, this makes it hard to rush early. That said, it is a good item to rush on champs that benefit from auto-attacking in general (Teemo, Shyvanna, Irelia, etc.)

Actually for Teemo, if you have enough to buy all of Wits end at once, I'd suggest starting on Bloodrazor with Pickaxe and Recurve bow instead because of how expensive it is, but how rewarding once it's finished.

Pretty much a bruiser item. I would say it is good on AD carries, except that with all your damage being physical, any armor pen being invested in is lost with this item. ADC's do not capitalize on wits end like other champions.