My sister's friend gave me her iPhone to fix. It's an iPhone 3G, so it's pretty old. I plugged it in to charge, left it alone for about 15 minutes, then tried to turn it on. Every time I attempted to power it on, it would show the Apple logo for a split second and then turn off. I tried putting the phone into DFU mode as well, but it just wasn't turning on at all, and the computer wasn't recognizing it, so I went on the assumption that it has a dead battery.
The repair process went very smoothly. I actually bought the "pro" repair kit from iFixIt about a week before this repair, as a "just in case" thing. Luckily, it helped tremendously.
After following the online guide, I was able to replace the battery fairly easily. I was actually not very confident in my work, since this was the first time I disassembled an iPhone (I've built a few desktop PCs in the past).
After replacing the battery and reassembling everything, it worked! The phone turned on and stayed on.
The spudger is really an invaluable tool for this type of repair!