I've bought both Joanas horde levelling guide and Brian kopps alliance levelling guide - and found that I really didn't use them after a while.

It kind of took all fun out of the game for me. Go here do this then go here do this then go here etc. I tended just to use them to find out where I should be at various levels (and that can be found without paying for it).

I did use the undead levelling guide a free guide at wowmb.net . Again I didn't follow it word for word but it works. (up to level 60)

Personally I like questhelper - grab up all the quests in an area then look on the map to see where to go (checking to see ones where multiple quests can be completed in the same area and off I go).

It's less intrusive than stopping to read a guide in another window.