well since I happen to have strong views on this, I guess I will make the first noise. Let me start my saying that I personally think that the multi-scale optical flow enterprise is a pipe-dream and it is time that people working on it realized the reality of the combinatorial structure of the problem. The general problem of scene matching does not allow for any of the assumptions that the people who use direct methods make. The brightness constancy equation is barly valid in anything but the most restricted of the circumstances, and a search enterprise which is based on it somehow does not make too much sense to me.

We are now in a time and place where there is ample computation power available to us (that is not to say that we should abuse it) to be able to be look at the combinatorial structure of the problems before us and design algorithms which explicitly deal with it.

The only thing which the direct methods people can claim over the feature based methods is that they use the whole image where as the feature based enterprise only uses "corner features". This makes a lot of sense when you are dealing with objects in the scene which have smooth boundaries e.g. an ellipse with a large enough radius of curvature.