I have been wanting this book since I first read about the content and so, on the back of a few commission paint jobs, bit the bullet and ordered it. As a massive SM fan I have to say that I'm very happy with the book as it is almost exclusively SM content cover to cover. The format is gorgeous, being a bit larger than the 40k rulebook but with less pages. Still, it boasts over 250 pages and, although not as lavish as the horus heresy rulebooks, is still a quality hardback format with full colour throughout. £50 is a bit of an ask thesedays but with the 40k rulebook and codex/supplements going up in both costs and quality, it is no surprise that the once proportionally super-expensive FW books (and models) are becoming better value for money compared to their GW counterparts. Indeed, page for page the book is better value than your standard RRP codex.
The first 25 or so pages detail the adeptus astartes and their manufacturing capabilities, with a few timeline pages of notable battles also. Then we get to the meat of the book - the rules. These are handily divided up into sections for handy reference. I love the fact that all the vehicles now have cross-codex consistency and that any units previously thought to be overpowered (lucius drop pods for example) have been brought in line with the rest of 40K. Speaking of which, each rule entry now has one of four stamps on it; Warhammer 40,000, super heavy vehicle, super heavy flyer or Lord of war. Some have more than one - a Fellblade is both super heavy and a Lord of War for example. This combined with the fact that the book cover has Warhammer 40,000 expansion printed on the front and a handy section on page 6 makes it clear that these rules are for use in normal 40K - given that they have clearly gone to some effort to bring both HH and FW stuff into line with the most recent editions of the rules/codexes I think this is something to be celebrated.
Some units get notable mentions, such as Baal predators, but the rules are left to their own codex. Then there are some vehicles that are excellent additions to Codexes that are lacking in certain areas. Missing a Chaplain Dreadnought from that Black Templar army? Well now you can have one in Elites offiicially. Wish Dark Angels still had exclusive access to mortis dreadnoughts? Well, they don't have exclusive access but every other army that can take them is limited to 1 per detachment whereas DAs get them as standard in Elites. (good way to compensate for a lack of air support) While on that topic, if the nephilim doesn't quite do it for you why not take a fire raptor instead? The book is replete with new options to pad out SM armies and add diversity. This is something I welcome and think should be celebrated. So long as your opponent has a chance to look over the rules beforehand there is nothing obviously game breaking in here, unless you count lords of war for escalation which is something non-specific to this book.
Other nice touches in the book are the additions of FW models in a more official capacity, namely the specific grey knight/inquisition vehicles and the odd SM model, such as the razorback infernus, which now gets its own entry. The Grey Knight models are for the most part simply formallised builds of the LR redeemer with psybolts and psyflame and the psychic rules added. There is a thunderhawk entry similarly with the GK psycannon and psychic pilot additions. The inquisiton entries allow for vehicles upgrades such as truesilver armour on valkyries. There are also SoB entries at the end, making this book effectively adeptus astartes + all other imperial stuff except obviously Imperial Guard. The very back of the book has a nice weapon and rule summary section also.
Overall I'm very pleased with the book as I have a few models that are contained within that I can now list-build with. My hope is that now all my SM armies are more or less complete I can add one extra FW item to each over time to add variety. A storm eagle to my Raven Guard for example. Although a hefty investment, I highly recommed the book to anyone who likes lavish full colour GW books and/or who has a SM army. If anyone has any questions about the book please ask and I will be happy to try and answer them for you.