Id like to correct this.
Kakkoiidesune does NOT mean "you're handsome"
It is actually translated more along the lines of
" ___ is cool," or "this is cool"
It's hard to describe it perfectly, so I'm sorry if I confuse anyone
The 'desune' is more of a confirmation statement. And 'kakkoii' pretty much means 'cool'
So by saying "kakkoiidesune" you are saying something along the lines of 'it is cool' or rather agreeing that something is cool. (Not cool as in cold)
Now, depending on what the conversation topic is, you could say "kakkoidesune" as in "he's so cool", but the literal translation is just simply stating that the subject which is being spoken about 'is indeed cool'
I'm not too sure if i explained this well enough, and if I am wrong anywhere, please correct me.