Having grabbed this at its release over a week ago and played it pretty solidly since then, I figured I'd take the time out to post a thread here about it.
Hearts of Iron 3, for those not in the know, is the latest Grand Strategy game from Paradox Interactive, makers of Europa Universalis, Victoria and... well Hearts of Iron 1 and 2. Note that when I say Grand Strategy I really do mean it. If you thought Supreme Commander was an RTS on a strategic scale, prepare for a rude awakening because it's got nothing on Hearts of Iron.
The game is, despite what you might think from screen shots, real time and not turn based, and covers the years from 1936-1948, covering WW2 and three years either side. Before the "yarboosucks WW2" crowd jump on the bandwagon with their list of timeframes that are less overdone, please note that Paradox's games put together cover the time period from 1066-1968, as well as a few hundred years of the Roman Empire period. So whatever you suggest odds are they've already done that as well. An interesting aspect of the game (and indeed most Paradox games) is that you can play any country. And I mean ANY country. Obviously the big players are in like the USA, Germany, USSR etc, but if you want to wield the awesome military might of Uruguay or Bhutan, go for it. You might not be able to ACHIEVE much but the game won't stop you trying.
First, to get a few things out of the way:
1) The game is buggy as all hell. This is pretty standard for a Paradox release though and it's not actually unplayable (except for multiplayer where it desynchs like nobodies business) just a bit ridiculous in parts, like when Britain starts producing 12,000 Crude Oil per day (many times the rest of the world put together) for no discernable reason, or the French AI stubbornly refusing to budge from the Maginot Line whilst italy invades and conquers their country from the south. Going from experience though, whilst their releases are notoriously buggy Paradox do actually fix their games, so it'll be fixed some point.
2) No tactical battles. If you're looking for tactical battles, look elsewhere. That's what your commanders are for. You just run the country. You can still fight a war of manouvre and so on but just be aware you're doing it at a world map level not a tactical map ala Medieval.
That said, onto the good.
The tech system has been overhauled from HoI2, with the few big upgrades model replaced with many smaller ones, offering a much higher degree of customisation and variety in researches. There's also a lot more overlap between tech paths, for example upgrading infantry weapons now upgrades the weapons of your marines, paratroops, etc. Which kind of makes sense.
The army structure has been overhauled too, now resembling something more like a real command structure. Your basic combat unit is the division, made up of 2 to 4 brigades of various types. You then have corps, armies, army groups and finally Theatres, each going increasingly up. Each tier has different benefits provided by the commanders to units under their command.
What's most interesting perhaps is that every level above division can be set under AI control, attempting to complete objectives placed on it by the player. As the game has about four times the province count of it's predecessor this is probably necessary to stop you going insane. Insofar as how well this works, the general consensus on Paradox' forums is that the AI is actually quite good at Corps and Army level, with a few players noting that it made moves that were better than anything they would have thought of, but that it's poor at Army Group level and woefully bad at Theatre level.
Conquest has been redone a bit too. Instead of having to capture every victory point to annex a country, you now have to capture a percentage of them equal to their national unity. The trick is that you can lower national unity by other means too, such as strategic bombing, destroying supply convoys, or certain intelligence missions. So now the giant empires with the odd far flung victory point aren't unassailable. The flipside of the coin however is that a country that's defeated can opt to become a Government in Exile. Doing so stops them from being annexed (the attacker still controls the country and can still extract resources, manpower and industrial output, but only at lower levels) as well as allowing the player to continue playing after a fashion, controlling partisan uprisings and so on. If their allies reconquer the territory it goes back immediately under their control.
All in all Hearts of Iron 3 has a lot of long term promise if you can overlook the short term bugginess, and frankly it's damn fun even with them. I'm currently playing a game as Hungary trying to reclaim the territory of the Austro Hungarian Empire in the buildup years before the war starts and I'm having an absolute blast. Hope to see some strategically minded dolts playing this multiplayer in a few months when they get it fixed.