The capacity of my battery had decayed to below 1000mAh.
Followed the replacement guide, and being sure to keep the screws and brackets organised, it all went very smoothly up until it was time to actually remove the battery. The adhesive strips broke almost immediately so I tried using a heat gun to loosen it and running dental floss and fishing line behind the battery. Both the floss and the fishing line broke on multiple attempts. I ended up just yanking the battery off the back of the phone which severely deformed it, as seen above. Bit dangerous but it worked.
All in all, the process took about half an hour and the phone works perfectly.
The iSclack was impressive, it opened the phone in seconds and only opened it enough to not damage the digitizer and or home button cables.
Also take extra care while you're removing the adhesive strips, try to go slow, maintain tension and move slowly around the edge of the battery. Chances are they'll break anyway but the process is a lot easier if you can get the strips out cleanly so give yourself every chance of doing so successfully.