Half of the point of this book is that the Allies won World War II because they were able to overwhelm the Axis powers with the sheer weight of their industrial production. The Germans had no chance of being able to produce tanks, guns, planes, fuel, whatever on a scale that could keep them on relatively equal footing with the U.S., the Soviets, and the British. Ellis has a lot of facts, and figures to detail just how outmatched the Axis powers were, to the point that, even when it looks like they were close to achieving some sort of victory, they really weren't. For example, during the Battle of the Atlantic, between the U-boats and Allied shipping, even when the Germans were sinking a huge amount of merchant ships, Britain was apparently still building up their overall merchant fleet (from ships they'd added from other nations or countries, as well as ships they or the Americans built), the Germans were still falling behind. Especially because Hitler didn't put enough emphasis on building U-boats, and even when he did, they couldn't build them fast enough to tilt things.
The other half of the point of the book is that the Allies could have ended the war quite a bit sooner if they had been smarter about using their material edge. Ellis details repeatedly how the Allies largely fail to use any real inventiveness in their tactics, and often fail to show the necessary urgency that might have enabled them to capture large quantities of German soldiers. So the commanders frequently seem perfectly fine with simply throwing wave after wave of tanks at entrenched positions, except at times when they could have made a major breakthrough, which is when they always seem to lose their nerve. The moments when large advances are made is typically when the Germans have decided to fall back, and so the Allies are merely taking land, but not really disrupting what effort the Germans can make to fight by capturing their forces.
In the Pacific theater, Ellis criticizes the Army and the Navy for being unwilling to work together (or more accurately, both being unwilling to be under the other's command) and go with a single advance, with Ellis arguing for MacArthur's idea of cutting through New Guinea and the like to cut off the supply of raw materials to Japan. I think it's worth mentioning that just because MacArthur said his goal was to cut off Japan's supply lines, that doesn't mean that's what he would have done if he had been given overall command (which, had they settled on a single approach, and chosen that one, there's a good chance he'd have been given the overall command). He said he was planning to move around islands that possessed strong Japanese forces and simply cut them off to die on the vine, but he still attacked well-defended islands in the Philippines, even when told to leave them, as Ellis mentions. So there's a big difference between what MacArthur says he'll do (or says he did), and what actually happens.
It does come off as a lot of Monday morning quarterbacking, something Ellis acknowledges as well, but even acknowledging can't hide the fact it gets tedious to hear again and again about how Allied commanders were unimaginative screw-ups who seemingly always make the wrong choice. They're too cavalier with their men when they shouldn't be, and too cautious with them when they shouldn't be. Maybe he's right, especially in the situations where Allied commanders had access to decrypted communications and knew their opponents' strength and intentions (such was the case for Montgomery when plodding after Rommel in North Africa). But it's hard to believe they were always this consistently fouling up.
Besides that, there isn't really anything substantive in here I haven't read in any number of other books on the topic. Overy's Why the Allies Won, for example, cover a lot of the same ground in terms of production. This is a nice way to have a lot of the relevant numbers in one place, but it's not new ground.
'In short, Hitler was not faced with a sort of military IQ test in which the correct sequence of binary decision-making would lead to the correct answer, but was trapped in a maze in which every option was ultimately meaningless because all the exits had been blocked behind him. His military incompetence in the Barbaroosa campaign was revealed not so much by his inability to get out of the maze, as by his ever having allowed himself to be immured there in the first place.'