The original battery was 4 years old and only holding a charge for a few hours. I could have had a new 3G from an Apple store for $79 as a swap or attempt to fix it myself for $20. Since I had a $40 Skinz sticker cover on the phone along with tons of apps and the time to restore everything, I went for the repair of my existing device saving me lots of time and money.
I have familiarity with electronics along with mechanics so I found this repair very easy. It took me about ten minutes and that is only because I went slowly step by step with the iFixiT video to ensure I didn't break anything with it being my first time tearing this device apart.
The only difficulty I found was during reassembly. One ribbon connector was a little tricky to slide into its socket and then snap in the other two connectors all of which were attached to the front half of the phone and gave little play. With a little finesse, it went together fine.
If you haven't messed with electronics or tinkered with things, be careful with this repair as small electronics are fragile and can break or tear easily. Make sure you have a clean area and be cautious with static charges. If you tend to break things when you take them apart, I wouldn't recommend doing it yourself. It's not hard, but it's not easy without some experience and I wouldn't practice on an iPhone.
The tools that come with the battery are plenty sufficient. I wasn't sure if I needed better tools when ordering and was happy with the kit tools. You will need a suction cup though which most people probably have with a shower/bath tray. Of course to get the sim card out you need the original iPhone tool or a small paperclip.
For $20, you can have a like new phone again in just minutes! That gives you time to eat that piece of cake!