Received my copy yesterday and had a good flick through.
I would agree with Geoff_B that the changes have made a good book even better. The plan drawings are reproduced larger so are easier to see and there are some new ones too. The model photographs are good (though at times the colour reproduction seems a bit garish) and the good quality and rare photographs really top everything off, one fault with the original volume was perhaps the lack of photographs to illustrate the built aircraft. The colour section at the back has been lost, but I guess with modern 3D art it had dated and the wider photo selection (and colour throughout now!) also made it less relevant. Instead the two extra appendix on Specs and contracts are more useful.
The post-1970s section certainly seems longer, not had a chance to read it yet, but it clear the extra research has made the story more complete.
I would say if you haven't got the 1st edition, then this is a must buy, and even if you have the 1st edition, this is still a worthwhile addition as an update and its still complimentary as many of the photographs etc. are different.