Battery replacement not as bad as expected (but for one mistake)
Battery was down to 80% health, and battery life was getting noticeably worse. I didn't want to replace the computer yet (~$100 for a battery is a lot less than ~$3000 for a computer), and I wanted to avoid having Apple do it (since I didn't want downtime).
Overall, the repair went fine. The instructions were easy to follow, and the tools provided worked as expected. As usual, detaching the little connectors on things was nerve-wracking ("please don't break, please don't break") but it all went fine.
Ironically, the only problem was that I accidentally ordered the wrong battery. I knew I had a "late 2013" retina 13", but somehow I accidentally ordered an "early 2013" ... which, of course, I didn't notice till I had the thing taken apart and was ready to put the new one in!
iFixit support was great: they don't do exchanges, but they just had me buy a new one and RMA'd the bad one for return. Alas, since batteries can't go on planes, it took about a week from when I started the repair till I finished. But, once I got the new battery, putting it in and reassembling only took a couple of minutes.
Make sure you order the right battery! And, look at all pictures and make sure you understand all steps. There were a few were I was a bit confused because I skimmed too fast, and (thankfully) went back and re-read.