I never owned a Megadrive and therefore didn't own a Mega CD addon either (being an Amiga devotee). However since then I've played quite a few Megadrive and MegaCD games via emulation.

Much like the Amiga CD32, the Mega CD saw a lot of direct ports from it's core MegaDrive parent, with the main differences being a better CD soundtrack.

There were some exceptions to this. However the MegaCD was quite late in the life of the Megadrive and the system's capabilities were really starting to show with the MegaCD. Look at the video quality in Nighttrap for example. Not good, especailly when compared alongside the 3D0 original.

Five MegaCD games were also released that required the 32X addon but I'm not sure how good they are as I've not tested them out yet.