Probably not, if your goal is better-quality telephoto shots. The community consensus is that clip-on telephoto lenses for an iPhone 3GS tend to reduce image quality: sample images are often soft, blurry, and low contrast. Because the phone has a small sensor, limited stabilization, and isn’t easy to hold steadily, you may also need very bright light or a tripod to get usable results.

In general, adding a low-quality optical attachment in front of a tiny phone lens won’t turn it into a strong telephoto camera. If you specifically want a lo-fi or toy-camera look, that kind of accessory might be fun. But if you want sharper zoomed images, it’s better to:

• move closer when possible (“shoe-leather zoom”),
• use the iPhone for subjects it handles well, like wider scenes,
• or get a small point-and-shoot / pocket camera for real optical zoom.

So: buy one only as a novelty or creative-effect accessory, not as a serious upgrade for telephoto photography.