Exactly what Seedling said. Riven Phoenix does veru good lessons, and teach you very well the general anatomy and a rules for construction, but in the end its no compromise for actually doing the studies yourself and getting a feel for how everything fits together. Also, Ive noticed that some of the heads he does, at least from the front look a bit unnatural. This doesnt really matter, seeing as it gets covered up with flesh later on - its just a base to work on. But if you're looking to get a perfect skull then theres other ways of getting it.
His lessons are excellent in getting a solid grounding on the human anatomy, and the techniques he uses for remebering everything and linking it all up have been used to great effect for centuries (just look at Da Vinci) but in the end you should try and do it youself
"Never have I seen a greater, or a more beautiful, or a calmer or more noble thing than you, brother" - The Old Man and the Sea - Ernest Hemingway