The iPhone 3GS has a pretty decent camera. Given the fact that is is also a phone means that more often than not whenever I want to snap a picture, this is the only camera that I have on me.
If you are into macro photography, there is a very cheap solution for you to get the most out of your iPhone (or any other phones too). All you need to get is a pocket magnifier/jeweler's loupe.
Jeweler's Loupe 30x21mm
The normal rules for taking macro photography will apply here as well. That is :
1) Keep your hands steady. Any movement you make will be multiplied many times over through the lens. Hold your breath before you take the shot.
2) Make sure you are in a well lit area. Usually taking macro photographs with your phone will involve you (and your phone) to be very close with the subject you are taking. This will sometimes block the light source and make the picture either very dark or grainy. Try to move around a bit and let the light shine favorably on your subject if you want to shoot without lighting aids.
3) Holding a magnifier before the camera lens is similar to manual focusing on a macro lens mounted on a DSLR. There is a sweet spot you have to be on the lookout for. It is a very thin margin between the subject being in focus and total blurring. Be patient and perservere. It gets easier with practice.