This is something I've wondered for a long time. Obviously the Sega Channel service ended in 1998, but I wanted to know if the adapter cartridge itself actually did anything when loaded into a Sega Genesis. I finally had the chance to try it myself! In this video, you'll see:

An actual Sega Channel adapter ("General Instrument" version) demoed in a Sega Genesis console.
A screen capture recorded from the adapter.
Closeup pictures of the adapter (outside & inside of the plastic shell, circuit board)

One thing I didn't really show in this video is that the adapter ALSO requires its' own separate AC power input. I don't have the original power supply, but I did have something close enough kicking around. Luckily, it worked!

Hope you enjoy the video!

Thanks to DUSTINODELLOFFICIAL for uploading the music.
Composed by: John Baker, Howard Drossin