You may ask "Why would I need a stylus on a capacitive screen which works so well with fingers?", but there are a few cases where a naked finger may not be available or adequate:
1) Long finger nails can make typing slow and inaccurate
2) Gloved fingers do not work
3) Navigating forms/spreadsheets and sketching can be painful when you can't see what you're doing (because your finger is in the way!)
HTC's own C400 stylus range however, though by comparison are very expensive and odd looking, do work very well on these screens - almost as well as a naked finger, and the target "dot" on the transparent stylus face-pad means that much better accuracy can be achieved than with a finger. I'd advise being a bit wary of sketching with these though as the pad as the end does seem to be somewhat unforgiving of dust on the screen looks as though it could cause scratches - perhaps not to be used without a screen protector?
The significant cost however does also have to be factored in to the purchase decision.