The most direct example of this is Nick Fury. Redesigned for the launch of the Ultimates, he is a dead ringer for Samuel L. Jackson, a point that was not lost on the actor himself and a resemblance which is referenced in the narrative fairly early on. Fun fact: upon learning of Fury's new appearance, Jackson struck a deal to secure the role for any future movies in which Fury would appear. That agreement has worked out better than I think anyone could hope.
The Tony Stark we see here also shares more with Robery Downey Jr.'s interpretation than the industrialist from Earth-616. Ultimate Tony Stark is a charismatic, fun drunk who flaunts his celebrity status to such outlandish proportions as conducting interviews during space flight with Shannon Elizabeth on board for the ride. With a slightly more realistic grasp on what's going on in this universe, Stark becomes a practical analysis on what kind of person would be crazy enough to jump into a suit of armour and fly around with reckless abandon. Perhaps that kind of person wouldn't be sober.
The other two members of the big three are markedly different here. Whether or not Thor is a Norse God is left more up to interpretation. Despite his strength and giant hammer, there are no firm indications that he comes from another world. He may only be crazy. What Thor is - for sure - is a left-wing activist who only agrees to help The Ultimates in exchange for concessions from the Bush-era government. That's a neat twist.
Captain America, though, has the most intriguing character tweak. When Steve Rogers first emerges from his icy stasis, there isn't much to distinguish him from the squeaky-clean Cap we know and love. What we find out, though, is that THIS Captain America is a bit of a jerk and definitely a bully. While he is still as passionate about defending what he feels is right, this Cap oversteps his boundaries and his opinions are not exactly virtuous. There is a legendary panel where Cap, faced with demands to surrender, asks "Do you think this 'A' on my head stands for 'France!?'" Original Steve would never, ever say that. Marvel made the correct decision by having its big screen Cap reflect the more traditional version of the character, for sure, but that does not lessen Ultimate Cap.
Characters like these are not worth much without a good story, and Millar delivers the goods for sure. He uses semi-traditional tropes: the team takes down a monster (a Bruce Banner who Hulks out on purpose to unite the Ultimates, killing hundreds of innocent people in the process) and fends off an alien invasion (the Chitauri, who are the Ultimate versions of the Skrulls.) What makes these storylines more interesting, though, is that they are nudged slightly into a more realistic setting with more realistic repercussions. It's reminiscent of Kurt Busiek's "Marvels" in that sense, which gives a more human perspective to major events in Marvel history. Only - in this case - it's the superhumans who are more human, full of flaws and making mistakes.
Furthering along its cinematic influence, Hitch draws "The Ultimates" like a blockbuster. There is a lot of wide framing and dynamic poses. The style of close-ups used is also very cinema-esque. Every hero comes off as charismatic (with the deliberate exception of Banner.) Colour tones shift from situation to situation, highlighting red in some, blue in others. How influential was this work? Well, Joss Whedon himself directed parts of the Avengers film to resemble Hitch's art.
To that end, anyone who enjoyed the Avengers film (ie: nearly everyone) would enjoy "The Ultimates." It has an air of familiarity that fans of the cinematic universe would pick up on, and the key distortions made to some of the characters provide for some thought. I've never been huge on Ultimate Marvel, but this has a worthy place in my collection and should be the first Ultimate book you grab.