To say that the classes are useless like 100% of the posts below my original reply is a touch harsh in my opinion.
Are your airlines going to teach you everything you need to know? You betcha.
However there is something to be said about having familiarity with the subject matter instead of going in completely blind. I will freely admit that I didn't use a word from the international class during my career at the regionals. I took the course because I was interested in it and knew it would be a while before I would have the opportunity to learn it at a major. I don't regret paying the extra money for that introduction as the course was very enjoyable for me.
As I stated before, the investment in this INTRODUCTORY class paid massive dividends when I finally did reach a major. When words like ETOPS, NAT, Eurocontrol and international NOTAM format started coming out of international initial class on day one, I looked around the room and saw glazed eyes, pale white faces, and people thinking they couldn't do it. I on the other hand had a very basic grasp on the material as I was introduced to it and that little bit of exposure made my life during international training much easier.
The dispatch schools that offer these courses do not claim to qualify you as an international dispatcher with the class, they are very up front that it is an introduction and that your airline will take you the rest of the way. Ultimately it's up to the student. I am sure I would have gotten by in international initial at my airline just fine had I not taken the class, but if I were to do it all over again I still would have taken the class just for my own benefit.