Has anyone had any experience with these passive converters made by Ultralink and Entech? Are there any other ones out there that are better? Thanks.

Yes, there are better units out there, but not until you reach the $150-up level. I commonly use the Ultralink version (prefer the one built into a 2 meter cord, composite on one end, s-vid on the other) for systems that are mostly S-video, but have one dastardly component that is composite. It simplifies A/V switching through an A/V receiver to go all one video type (in this case, s-vid).
Do not mistake these units as a way to upgrade your video quality, they are not, but do not seem to appreciably downgrade quality. If you need longer cable length Ultralink makes a little inline pod that you attach your own composite & s-video cables to...tape your connections as the module does not always attach snugly, and we don't really like crawling behind gear when we don't have to.

We use these very frequently. They work great. Same as Ian, they do not improve picture quality but allow you to use S-Video in applications where you would not normally have all S-Vid equipment.

Faroudja VS50 is probably the best active unit. Overpriced for just Y/C conversion as it also performs YCrCb converstion.