The Extreme leveling, was a pay guide.
At least at the start of WotLK, when I got it.

Compared to the Brian Kopp guide, Extreme was noticeably faster on the 2nd run through.
But did not give nearly as much information as the Kopp guide.
So the first run through it, I spent a lot of time looking for things.
The stacking is more efficient, if you know where to go for each next step.

The free guides, like Jame's is probably the way to go.
From what I've seen, there might be a slight difference from one guide to another.
But generally speaking, the pay guides cost 20-30 bucks.
And aren't really that much better.
Although some of them have an in-game addon and mark your map etc.
But Questhelper or Carbonite can do the same for free.