Doable DIY repair, but watch out for adhesive battery strips.
My phone was getting slower and slower, and the battery was just barely making it through the day. After Apple revealed that they were slowing down phones with old batteries, I decided to get a replacement. But Apple refused to replace my battery because my phone apparently had water damage (not enough that I ever noticed). So I tried an iFixit kit instead.
The kit is really great. I'm impressed overall at how much you get for the price. The repair itself is certainly tricky and requires some patience, organization, plenty of light, and a good pair of eyes (and/or a magnifying glass). The repair took me about 75 minutes, but over half of that time was spent trying to remove the adhesive underneath the old battery. If you snap off one of those little strips, then it's very hard to remove the old battery. I used a hair dryer and a plastic wedge designed for removing body panels from a car, and it still was a painstaking process.
Go slowly. This repair is easy to screw up. And when you get to the battery strips, put the photos aside and watch the iFixit video for that step.