Simple to play? Depending on what you compare it to, it depends. I'd say it's rather complicated, but less so than some other games from the same company. But that's not saying much, since Paradox's game are a lot more complicated than games from pretty much every other company. Let's just say it's the simplest of their really really complicated games.
Easy? Depends on which nation you play as, and what difficulty level you play. Play with France on an easy setting and you can cook a stew while winning. Play as a backwater north american nation and you will have a lot of trouble achieving anything. Besides, "easy" sort of implies you are trying to win, and there isn't really any victory in this game. You just play 'til the end of the timeline and that's it. You gotta set your own objectives.
Worth getting? Yeah, but at this point I'd say it's almost imperative to get it with all the expansions. Don't bother with just getting vanilla.