I can totally see why Jon Favreau wanted to base the first (and also the third) Iron Man movie off of this book. It's approached about as realistically as possible, with some fantastic, grounded character work for Tony Stark, and big, cool, tech-centered leaps forward for Iron Man. The Iron Man origin story in the first Iron Man is almost exactly the same as the one here, along with the same strong parallels to the War on Terror and our roles therein. 
 Beyond the similarities to the films, this is just a flat-out good Iron Man story. The enemy is a fairly random everyman, rather than a big, suped-up monster or or some kind of Super Brain, which lends the story a groundedness that Iron Man can sometimes lack. There's a lot of weight-of-the-world kind of stuff with Tony Stark worrying about his culpability in wars, but it never reaches a point of feeling too burdensome. It just informs his decisions, as it should 
 One of my few gripes with this story is with the technology and bionics set up in it. Ellis goes through such pains to write a realistic, modern story here, that I was a little disappointed that the huge advances in tech (and I'm talking mind-numbing, MASSIVE advances), are given barely any explanation. They're just kind of stated as being possible, and then they happen. I'd be okay with this if they didn't then rely heavily on those same advances to further the plot along AGAIN, and in so doing create a variation on a technology that is just flat impossible based on what we were told about the first iteration. Look, this is basically impossible to describe without spoilers, but just trust that the technology is used as basically a literal deus ex machina instead of as a thoughtful use of technology that could potentially exist. 
 In any case, like New Avengers, which started right around the same time as this series, Extremis is probably most notable for fully brining Iron Man into the 21st Century. It changed the armor, it changed the character of Tony Stark, and laid a solid, modern foundation for later writers to build upon. Matt Fraction owes a lot to Extremis for allowing him to do the incredible things he did with the series further down the line. To sound like the back of a book jacket, this is a must-read for Iron Man fans for sure.