Long reviews tend to mix in their opinions too much to the point that it's more than what is necessary. Short ones are obviously concise and the reviewers won't bother wasting your time with too much special effects.
Anything beyond 10 minutes is too long for me. Angry Joe is an example of this. His review shows are too long for their own good.
But I tend to favor written reviews and look up gameplay separately. Why? Because I'm fairly impatient and can get to the points important to me faster in a written review.
That said, if a video review uses gameplay footage well to enforce the points the reviewer talks about, I very much enjoy watching that. Basically, if there's a problem, the video shows that. If something is entertaining, the video show that. At the same time as the reviewer tells us about it.
That's one of the reasons why I do watch Gametrailers reviews, in spite of their tendency to spoil some plot events. You know, with their tendency towards editing in in-game dialogue to agree with the points made (Or otherwise comment them), after or before showing the thing spoken about.
Bugs? Show bugs. Poor controls? Falling off a ledge. Good combat? Rapid fire montage of enemies being blown to bits. Gorgeous environments? Stand in a good spot and pan the camera across the environment. It's entertaining to watch.
Know that if you allow someone to take advantage of you, they might make it a habit.