Learn English – How bad is the use of “n***er” today

Pretty bad!
I remember I was with an African-American friend in line to get into the movie hall and I said it(the color) out a little too loud and the whole area went silent with everyone trying to avoid eye contact. It was pretty embarrassing. Don't even want to think what would have happened if I had used the n word instead. :(
Besides here is the definition from Oxford online dict:

nigger: noun
offensive. a contemptuous term for a black person.

That should suffice to ans your first half of the question.
And for the second half, it totally depends what mood or what group is it used in. Among friends, it doesn't matter. You will see African-Americans use it in a joking manner all the time. Also, you can hear a lot of rappers using it too. Nor will it matter if an old African-American uses it. ;) But these are examples from the States as from the this wiki entry on African-Americans.

Also, check out the OED definitions for the word Nigger, which also accounts for its usage/meanings from a wider global perspective.