There are some great improvements, but also some severe deprovements

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The Intelligence is neat, but frustrating. Much more realistic to not know everything about another nation, but you have to send spies every week and you miss many weeks, like with influencing. Also is very difficult to get a few spies in.
The sliders are a great improvement, but require more attention to making runs of things, I never know when a series stops now. But that is no problem.
The AI seems good, but I am only in my first year.
Tech trees a tiny but expanded, but not much.
Ability to uprade/reinforce or not is great, almost the only reason to even buy this game. (Along with the sliders)
Escort fighters are now a brigade, not sure if this is better or not, now I have to attach one to each bomber, kind of a pain, but will save on a slot for another bomber or two in each group.
The new leader rules are horrible. If I had known about this I WOULD NOT HAVE BOUGHT THIS ADD-ON. It has destroyed my enjoyment of the game. I will most likely go back to original until/if this gets fixed. Your bombing leaders gain no experience! Oh, unless they are in an air battle, in which they will lose since they are bombers. Also, only the leader in charge of the battle gains experience, now none of your leaders will go up in skill level since FM's take forever and noone else will gain any experience at all.
The auto promote seems broken or ruined, I have only one FM and a couple of Generals, I have to promote myself, but because of the above, noone has a very good skill level.
Convoys do not work as well as in the original. I have had to fix several because they would not deliver oil or suplies when I had 1800 convoys not being used.
Overall, I would not buy this again. There are some good things, but there are things that were ruined that didn't need to be touched at all. I love HOI and spend way too much time on it, lat least thats what my wife says, but I am very dissapointed in this add-on. The good thing is that Paradox is such a great company that they will improve things over the next year, I just hope it fixes some of these that I mentioned.