what are the pros and cons. I would like to know beocuse only KK and Jeff hanneman use them. And how good does it stand out to a FR. And please dont post the youtube comparisions by dmometalguitar please
I'm afraid not ONLY Kerry King and Jeff Hannemen use them.

Quite a few people use them, if only two people used them they wouldn't have a huge company with many different designs.

ANyway, they are said to be really good, essentially a locking trem version of the bigsby (where as the floyd rose is the lockin' version of the vintage strat style trem) as Pifty (from these forusm) will agree with (I remember his almost religious opinion on that )

Though also they've been compared to Ibanez ZR tremolos. Now if this view is correct (I'm quite a stranger to kahler) they'll be good, ZR tremolos are very very very good!
Kahlers have a much lighter feel than a Floyd. In general, they don't have quite the dive range and lack pull up range. They also require very little routing. Unfortunately, they are not the best at staying in tune, though this depends a lot on the guitar. They likewise have a lot of parts to them making it rather complex to work on one.

A Zr is simpler to work on, but parts are hard to come by. They do however rival a Floyd in range. They also are double locking. I forget if Kahler makes a double locking version of their flat mount trems. The ZR also needs a lot more routing and the quality of the materials is questionable. Same goes for lower end Kahlers.