In general, there are three opinions about this title song:
1. OMG OMG OMG, so flawless!!@!@!@!!!! Dying! SOOO PERFECT!!!
2. It's a grower.
3. OMG
Dissapointment, your name is "Kill "Bill"
In my opinion, I'm really glad they didn't release another song like SIXTH SENSE. I mean, since Abracadabra (and even before that) they always had this "diva" image and always posed and released things to match this "concept" they've been on since "Your Story" days (in one way).
"Kill Bill" is the first fun song they've ever released in their careers. They had some songs like "LOVE Action" and "Hold the Line" but "Kill Bill" is the first one that doesn't have all that pressure and concept to show them as superior ladies or the queens of Kpop.
They have already proved everything they needed. Now, it's time to relax and have fun. I haven't listened to the full album yet, but as far as I have, it's weaker than it's predecessor, but nonetheless its fun.
I'm happy with the direction they took