We moved to it as a whole school this time last year (Juniors). Each child has a book, and a session usually involves the teacher reading, and "tagging" children into read a paragraph or so and then the teacher taking over again, or tagging in another child. Once they get used to it (our new class aren't quite there yet!) it's fairly seemless - the children have to be following along all the time, and the teacher just says their name at the end of the paragraph, and they take over almost immediately. When the teacher takes over again, they don't stop them, just start reading over the top of them, and the child stops. Sometimes, the children read a page or two with their talk partners too.
We do comphrension questions every lesson, which the children discuss with their partners and then feedback to the class - we use RIC, so one question is just retrieving an answer from the text, one is inference, and the third is about the author's choice (ie why did they use questions - what effect does it have on the reader).
Once a week we do a written outcome - this can be written comphrension questions (new class are struggling with this, so we've had to do lots of modeling), a prediction, dictionary work, "agree/disagree/undecided" statements, where they have to state their choice and back it up with evidence from the text.
Reading test results were hugely improved from Christmas (only had half a term of whole class guided reading) to the end of the year. Year 6 SATS results were much higher, too.
As a year group, we have four books and half a term's non-fiction in a year (one book is really long and needs a whole term). These are rotated so no class is doing the same book at the same time. I do wonder how long the books will last, though - the book our class is reading at the moment is on its fourth read (read by all three classes last year and now ours this year) and we've already had a couple of books falling apart, and almost all of them need new plastic covers.