Ahh, the battle stylus...
I've read several reviews on this and all have been unanimous in saying: its an awesome upgrade from the stock stylus.
Whether or not its worth $15 (for some pretty pricey for a little stick, metal or no) depends on you. For example, are you really hating the current stylus for its lack of heft? Because the battle stylus will definitely help in that department...supposedly its tip helps with accuracy a bit as well.
Personally, I'm very tempted to try one out and may do so in the near future now that I have DS games that are really compelling.