I received my DAGi stylus a few days ago and it actually works pretty well. It slides well and has good sensitivity and is more accurate than I expected. I don't have to press really hard like I've seen people say with other phones. The transparent pointer is a really nice touch. It is kinda finicky about the angle the stylus is resting on the screen though, which makes sense since you're pointing with a flat disc rather than an actual point. You get used to that though. The stylus is all plastic. The build quality and feel seems somewhat cheap. The part where the disc is attached seems like it could end up breaking if you're rough with it. The Pogo brand capacitive stylus (the other popular brand) seem like they're metal and built a bit better, but I've never used one. The pointing tip on those is much bigger though so you may sacrifice some accuracy. Go ahead and look them up as well. They're sold on Amazon I believe.
The stylus works much better than your finger on certain games where precise pointing or drawing is required, such as Cestos, Space Physics and What The Doodle. It also works well for clicking small links and text when web browsing if you don't want to zoom a lot or use the d-pad. All and all though, for most other applications your finger is sufficient. I really just bought it for games. I'd recommend getting one if you think you regularly play these types of games. I wouldn't play What The Doodle or Space Physics without it.
They have a few different models, but they're not all in stock. The company has a website, but they only sell through ebay. Compare the model numbers on the website description to the ones in the ebay list. I got model P005 which is the full size version (P5B on eBay).
http://www.dagi.com.tw/ <-- Site
dagi stylus, great deals on Cell Phones PDAs, iphone stylus on eBay! <-- eBay
search for "dagi stylus" on youtube for some demo videos. I think there are also pogo stylus demos as well.