I have seen combo videos done well before, far better than anything I've ever seen on text, but videos can be difficult to watch if it just focuses on quick gameplay and not on how to actually perform the combo, as so many often do.
The greatest instructional video I ever watched on how to perform a combo was actually for the Virtua Fighter series. They started with an in-game shot of the combo, then a shot of the hands using the arcade controls to perform said combo. While it did take a little working out, it was easy to get an understanding of the timing by watching theirs. The video also ran through the combo multiple times, and also shorter variations, helping to "build" it basically.
For instance, the first hit was a pop-up, then a quick series of punches, and the video would show that and the controls. Then they would redo it, adding a kick or something at the end, and then show the controls again. I watched the video twice, then sat down in practice mode, and about ten minutes later I knew it well enough to actually pull it off while playing the CPU or other players.
Plus, since it had helped to establish the flow of the game, I started figuring out variations that added more to the combo, little tweaks that work better on some characters than others, or alternate means of performing nearly identical combos based on certain situations. It definitely improved my game, because I felt that it was actually doable, unlike many of the other videos I've watched. Those tend to come off more as a "watch how good I am" video, as opposed to a tutorial of some kind.
So yeah...I'd say most effective is a combo video that actually intends to instruct.
