Well, as someone who has learned from DVD's and also posted my own meager stuff, I have some thoughts on this. Never mind the idealistic and naive fluff I put in the 'Self-Learning' thread.
First of all, learning a form from DVD is rather easy. Two punches and a kick, turn this way, block high, low, hook, uppercut, blah blah. Film yourself or have a friend look and compare to the DVD in question. "He/She did it this way, not that way, remember the stance, blah blah."
With diligence and practice you can make the form look nice. Except you really don't know what you are doing or why. Well, some parts are pretty self-explanatory, and if there are applications on the DVD you have a starting point. Posting it and getting comments and advice is also VERY helpful. But you need someone to tell you the little nuances that turn a weak technique into a solid move. Someone who knows what it's about and can correct you.
Here is what I feel today as opposed to when I started the Self-learning thread: DVD's and learning forms is very helpful to get a framework going, so that when you have the opportunity to visit with a teacher or someone experienced you have something to work with, instead of starting from scratch. For those of us who don't have regular access to a school, they are a great help. Just DVD's alone aren't much use.