Boz,
5x5 is not a magic program, it has a following of people who love it and rave on about it. There are a few different versions of the 5x5 program - Madcows, Starting Strength, Strong Lifts to name a few.
I have done it, I got a bit stronger yes, looks went down hill, felt like a lazy arse workout though. I was tired at the end of every day yes, but doing 3 exercises on any given day was hardly overally difficult.
First few weeks of the program I was finished the routine in 30 min. Longest I got to was 1hr and 15 minutes.
It serves a point, but I'd consider it to be a beginners program. Last page Markos said who designed the program and why.
It was for PE Teachers to give to high school students because they had no idea about weight training. There are also a lot of people out there who argue it was more specific for high school footballers who were already strong, but that is debatable.