I don't really play slap - but I have that DVD. IMO it's not a great instructional video on how to actually play slap bass in detail.
It does have a lot to offer though in sort of a broad sense. The guys talk about health with your hands and the problems that playing that style can cause if you do not take proper breaks and such. They give tips on the motion of slap, where on the finger board, string height, etc.
They also run through examples of many different genres and how slap lines relate to them. This part of the DVD is very cool.
If you are serious about rockabilly slapping, join this forum:
http://www.doublebasschat.com/forum/index.php
There are guys over there that are great at it, and are always willing to offer up advice. In fact I think the guys that made that DVD are regular forum members.
Also check out this:
http://scotthinds.tripod.com/
