Yeah, EU and HoI series are both made by Paradox, so they share *some* similarities. HoI2 is much much more user-friendly (in terms of interface and all) than its predecessor. I'd imagine EU3 (2007) will follow suit as well.
As I said, HoI2 has a demo available, although all you have in there is combat, no production/research/diplomacy.
I mean, come on, diplomacy with 100+ nations. It doesn't get any better than this. That's one of the big draws. The whole world is included, not just Europe or the Pacific. If the nation existed back then, you can play it.
In terms of learning curve, there's a tutorial which explains the basics pretty well. With just that, you should be able to jump into the game and start playing (though, playing the game will probably be a learning process as well). "Easy to learn, difficult to master". Not to mention a pretty resourceful manual.
HoI2: Doomsday is a STANDALONE 'expansion pack' that includes everything in the original HoI2, and adds additional scenarios, extends the timeline to 1953, etc. Thanks to its xpac status, the price is also lesser (20$ US).