What is your general opinion on cover songs ?

i am a big fan if an artist adds to an already good song, it often is even better then. it sucks if its just a copy and sounds exactly the same... it must be drastically different.

like this Patti Smith cover of the infamous Nirvana song Smells Like Teen Spirit

[youtube=https://www.youtube.com/watch?v=M_ciiCyxOJA]