Although super happy with my Iphone experience, I had picked an Iphone 4 with a loosy mic. People won't hear me well and I had to either yell at my phone or use headphones.
The repair required opening the Iphone and changing the dock connector, where the main mic is. I decided to change the battery as well to give my phone a bit more juice.
The repair was very smooth. All I really needed was to follow the video. One bit of advice that is missing in the video is that you need to remove the microphone rubber enclosure and put it on the new dock. That info is only on the step by step html guide.
WARNING : I don't recommend this repair to people that have no experience in precision work. Some parts are VERY fragile.
One thing, don't forget to pull up the home button flex connector when installing the new dock connector.
Otherwise you will have to remove the dock you just installed.