When it comes to online courses you will always find as many negative reviews online as positive – only you can decide which course sounds right for you.
Of the three you've mentioned, I have only heard of iCAL but that's probably because one of their people pimps his courses in response to almost every thread on every TEFL forum I've read! Personally I took an online course with i-to-i. I was very happy with it and had an extremely helpful tutor. I believe however that the format of the course has changed since I took it, and that you no longer have one dedicated tutor. Instead your work is marked by whichever tutor gets to it first, which may or may not be helpful.
As mentioned on your other thread, 'Headway' is not a method but a series of coursebooks published by Oxford University Press. It is therefore not something you will be taught on a TEFL course. You might find it helpful to buy one of the coursebooks to see how the lessons are structured.