My battery has been acting up for months, so it was time to finally do something about it. My local Apple-authorized repair shop wanted $440 to replace the battery. The nearest Apple store (1.5 hours away) wanted $199. In both cases, it was going to be 3-5 days without my computer. As a doctoral candidate finishing a dissertation, cost and time in both cases were certainly less than ideal.
The repair went extremely well. I followed the guide as described, didn't skip any steps. It took me three hours... and was actually a lot of fun. The repair can certainly be done more quickly, but this was a new experience for me and I was extremely cautious in following the steps. Everything that was disconnected in the process (fans, mic, audio, etc.) are all working.
The only headache was removing the adhesive between the battery and the computer casing... and worse, removing the little scraps of adhesive on the body of the computer after removing the battery.
Definitely keep track of all the little screws. I used a handful of notecards to label the size, color (as shown on the guide), and step the screws came from. So a card might have something like "4.4 mm, Red, Step 13" and the corresponding screw was placed on that card. It made remembering all the screws super easy.
Also, there are definitely some small details missing from some of the steps. So check out the comments left by other folks under each of the steps (e.g., step 24).