Should I buy a new or used Rocksmith game for PC?

Would there be any difference? I've read that it comes with an "Activation key code." If I bought a used copy, would I be able to reuse the code? If not, what features would I be missing out on?

?2015-02-14T12:03:07Z

DO NOT BUY USED PC GAMES.

Activation code is one use only.

Surveil Deez Nuts2015-02-14T12:09:11Z

So what do I really need the activation code for???