So my 6S is actually one of those that could have received a free replacement from Apple had i just taken it to the store. My issue with this though is that the current iOS is not jailbreakable and it was more worth it to me to use a crap battery on a jailbroken iDevice that stock iOS, straight up deal-breaker. But after not even getting half a day out of the battery and seeing the capacity drained down to 30% and as low as 5% i decided it was time to do it myself.
It went relatively well. I read every step and looked at every picture of the how-to extensively so i knew what to do at each step. The only issue i had was one of the first steps but it might be my own fault as I didn't remove my screen protector when applying the suction cup because i didn't want to replace it. So it took a few tries before i was able to get the screen to pop up slightly but once it did, everything went according to plan.
The only other issue i kind-of had was applying the new battery adhesive strips. I got them on the battery fine but i was worried about placing my fingers/hand in the wrong spot on the iphone itself when applying the new battery that i ended up putting the battery in somewhat crooked but it still plugged in and closed up nicely so all in all no big deal i guess and i have to say, hitting the power button was scary as !&&* the first time because i didn't know if i did it right or not but once i saw that apple logo and the new 100% capacity i was through the moon. Lasted me almost 2 days on a full charge, THANK GOD FOR IFIXIT!!