In 10 minutes I could get the GIST of it, not have it down. An hour of speedreading would accomplish the same thing. In 50 minutes (10 minute preview and 2 20 min superread/dip/skitter sessions) I would be done with the whole thing for good. Your question was "is it really that much faster than just reading the material straight through?" and the answer is- it's faster and you have it understood more thoroughly because you're doing something very different than just starting at the beginning and reading all the way thru to the end. Your going thru multiple times at high speeds and even if you finish with rapid reading you'll have your old time beat plus have more of it remembered. If you slow read something you are still going to have to go back over what's important to you and PR does this in it's steps and in less time. As Lomas said, you aren't going to activate everything you read. You may go thru a book and find nothing that you need- just think if you were to have slow read it.
[is the comprehension you talk about, an overal view of the material, or comprehension detailed enough to write an essay over?!?!]
An overall view can be achieved very quickly. The level of detail that you want from a book is going change as you read it. You might want to get more into certain parts of the book and less into other parts. I've found very few books that I want to comprehend 100%. Even with excellent books such as "The Einstein Factor" I find I'm blowing by much of it because I either already know it or I'm not going to do it and I quickly get into what I will do. If I were to slow or speedread I'd still have to read those parts= wasted time. Thanks to the PR step, purpose and mind probes I know what I want and where to get it. If I were going to have to explain the book or do an essay I'd throw in a 10min mind map.