I know that VGA is usually the preferred video signal for Dreamcast, but I like playing on a nice big CRT which doesn't have inputs for that, and S Video is the next best thing. I've found that many off brand S Video cables have a checkerboard effect, an audio buzz, and just aren't much better looking than regular composite. The official Dreamcast S Video cable is expensive, and I didn't know of any good alternatives, so I took a gamble on a $10 cable by Performance.
The cable doesn't have any composite output, just an S Video plug and 2 RCA plugs for sound. Strangely, the S Video plug is colored yellow, but don't be fooled, as again there is no composite output on this cable, only S Video.
The picture is nice and clean with no dot crawl, much better than composite. There is no checkerboard effect to be found, this is "true" S Video and not composite coming out of an S Video plug. The sound is also good, with no audio buzz that you get on crappy cables.
Overall, the Performance S Video cable is good. If you're looking for an S Video cable for your Dreamcast, this one is recommended. If you're playing on a monitor or a flatscreen TV then you might as well just use VGA instead, but hopefully this will help anyone who was in the same boat as I was.
Here's a few samples using a capture card and this cable.
(Anyone who can guess all the games saved on my VMU above will win a grand prize of nothing)