To my way of thinking, Mark has nailed the problem right here. People who learn from videos cannot possibly have that deep, solid confidence in what they've acquired that way because there's an upper limit on how secure they can be about just what it is they have acquired. It's not all that easy to learn complex moves from a video—you think it's going to be, but it isn't; there's too much going on in any sequence of movements, and often very small and subtle point make the difference. I was reminded of this at the seminar on Combat Hapkido I went to over the weekend. One of the component techs involves countering a grab, to wrist or shirt, by covering the gripping hand with your own, folding it and twisting it, and then rotating it in at a particular direction corresponding to an angle where the wrist has almost no rotational freedom. There's no way uke can stay on his feet when significant pressure is appied to the wrist in this way, but you either actually have to be there to see what that angle is, or the video would have to spend a good deal of time and footage emphasizing the point. But the latter possibility is pretty much ruled out by the amount of other stuff that the video will cover. A good instructor, however, will make it clear with just a word or two if you're not getting it right; s/he'll see what you're doing and if it's ineffective, will correct it by showing you how it's done, in just a few seconds of instruction time. And then you'll know just what it is you're supposed to be doing. With a video, you're never going to be able to be quite sure, as Mark points out.
I happen to have a serious problem processing complex spatial relationships, or even simple ones for that matter. And a surprising number of people I've talked to have the same problem, though maybe to a lesser degree. For me, watching moves on a DVD that I don't already pretty much know is just baffling for the first half hour or so until I can work out what the people are actually doing, and even then, I'm none too sure about what I'm seeing. With an instructor physically present, it's a totally different ball game, and worth any amount of time and money, I think, if you're really serious about getting it right.