i can recommend to watch one dvd firstly, before buying some of them. i was very disapointed as i had the chance to watch some of them.
maybe it is just cause i thougt about some “real” training lessons. instead it was mostly “hey - have a look over my shoulder how nicely i can paint”. it was not more than one of thousand step by step tuts from the net just with much more steps 
not all dvds are like this, robertson teaches you the fundamental stuff - it is a nice course to learn perspective.
but for me it makes no sense to watch somebody (church, feng) 2 hours how he finalises just one painting. after 10 minutes it was clear how his approach is and i got bored to see him making the same stuff over the whole picture.
a highlight for me is the ian mccaig series. there you can really see how he creates a nice story in short time, designs the main characters, paints mood pictures and much more.
and he is a great speaker too.
another interesting one was the first huante dvd. there you can see how he developes out of nothing a strange creature just by playing around with basic shapes and defining them more and more.
such things were more interesting for me, cause you can see how they are developing ideas from nothing, how they think, why they are doing stuff like this and discard other ideas and all the interesting stuff you can learn from.
but thats just me, others may have more fun with all the dvds.
but a training course should look differently.
i want to know how i can practise stuff like this, what are the different procedures, what should be avoided … all the stuff people can train at home who have not the chance to study it.
instead there are too few informations to be really helpfull for most of the dvds.