Vastly improved picture comparing to composite AV cable
It's not a big surprise that picture received with using this cable is much, much better than in (usually) included cables to Mega Drive. Better colors, more consisntent picture... I personally didn't heard too much improvement regarding sound, but it's fine, since the main reason of buying such cables is picture's quality.
Please, keep in mind that such cable (usually) won't help you if your TV is not detecting video signal from your Mega Drive (in such cases you need video converter - separate device).
In my opinion, such cable is one of the cheapest (but at the same time - efficient) ways to improve quality of picture from your MD console. Therefore I would recommend to check such cables before you will start to try with various converters etc.