The majority of reviews I have read about the HTC ST C400 stylus are aimed at people who do not want to have to take off their gloves to use their capacative touch screen phones. I do not wear gloves so this is not an issue for me. I am more concerned with the speed and accuracy gains possible when using the stylus over what can be achieved by using fingers alone.
The HTC ST C400 Stylus Pen is an official HTC capacitive stylus marketed specifically for use with HTC capacitive touch handsets. It will of course work with any capacative screen device. I tested the stylus on my HTC HD2 smartphone. The HTC HD2 runs on Windows Mobile 6.5. While Windows Mobile provides good support for productivity and desktop synchronisation, it is not very finger friendly and can be difficult to use on smart phones, especially for those with large fingers. I purchased the C400 Stylus Pen specifically to address this problem.
I have used it for a week now and unfortunately the C400 has failed to meet my expectations. It does work, but is is very cumbersome to use. It feels light, very insubstantial in the hand. Even with practice I have found it difficult to apply the "tip" at the correct angle to the screen to have the "touch" recognised at the first (or even subsequent) attempts. Also, the clear plastic mechanism at the tip of the "pen" looks very fragile and easily broken and so there is a concern about carrying the device in the pocket or gadget bag. In summary, it is difficult to be accurate with this stylus - every bit as difficult as using your finger - and therefore you gain neither speed not accuracy by using the device. As this is an HTC badged product for use with their smartphones, I had hoped that it would be well designed and highly usable. For me, this is not the case and overall I am highly dissappointed with the product. I would not recommend this product.
While my comments above relate totally to my personal experience with the C400 stylus, I hope you find them useful when making your own purchase decision.
Notes :
I have average sized fingers for a man and therefore I normally have some difficulty using touch screen phones(e.g. it often takes me two or three attempts to select the correct option in drop-down fields). I found that using my fingers provided a greater level of speed and accuracy than using the stylus. However, people with large fingers, who have great difficulty with using touch screens, might gain some benefit from using the stylus once they become accustomed to it.
The HTC HD2 Stylus seems to be a re-badged Dagi Stylus and not a device designed "from the ground up" by HTC for their touch screen phones. You may be able to find the Dagi stylus at a cheaper price than the HTC version.