I enjoyed the book, found the training hard and flunked the marathon - but I think most people on the P&D thread seem to have great marathon success by following the P&D plan whether they manage all the sessions or not.
I did 6.25 at 6:21 pace last year aiming for 7 I think but failed on the 5 and 6 and think I was only hitting about HMP even on the 4. I also failed on I think it was 4*1200 at 5k pace and they do give you a very wide recovery range of 50-90% of session length so you can make the intervals easier. I did succeed on 3*1 mile at 5k pace in my final run before my foot problem.
I think part of it is also about training your body that it can keep going when every sinew is telling you it can't but its not very pleasant!
As Madbee said the sessions are quite well spread out and P&D is mainly about lots and lots (and for some lots more) Easy miles. When I did P&D last year I probably didn't do it off a very good base so found myself naturally getting faster as I went through the plan and hence more able to do the sessions.
The ones I was the least convinced about were the 6 and 7 miles at approx 10 mile race pace. But some of the super-long reps at 5k race pace were a bit hardcore too. I was fine with the long MP runs.
By contrast, SG never gave me a session I couldn't do, and nor does my current coach (though it is a slightly different set-up as we don't have pace targets, but also he is there and can adjust the sessions as we do them).
This is also true for me. This has the added confidence boost that when you are given the session to attempt you know you are going to put yourself through a bit of discomfort to get there - under normal circumstances I achieved every session. I think the hardest session I can remember was 'Tempo madness' which was 2*2.5 miles at Tempo pace off 2.5M easy recovery.
Compare that to P&D where there were a couple of sessions where I was planning to fail before I started and some others where it was a question of me deciding at what point I stopped or amended the session which I don't think breeds a good mental attitude to either the training or racing.
It definitely got me fitter though but it was also the highest mileage I have ever run so it's not black and white.
As has been said many times before there are lots of ways to train and some get fast results at a higher risk than others that may get slower results but at lower risk. I don't think there's any harm, provided you accept the different risks, of trying a different training approach or training for a different distance every now and again.
I originally thought I was getting very fast results off low risk training but perhaps more recently I allowed others improvement to cloud my mind.