I always purchase the EU series and the HOI series when they are first released. Yes, they have always been buggy and somewhat limited upon the initial appearance of each series (EU3 and HOI3, for example). But after one or two expansions and then one or two patches after that, they always blossom into fabulous historical simulations.
The classic example for me was HOI3. HOI3 was a bit too buggy IMO when it first debuted...I think Paradox releases most of their games a bit too buggy in my view. Still, after the most recent expansion and subsequent patches, I'd consider HOI3 to be the absolute best game of its type anywhere...ever...it's really stunning. And EU3, after it's latest expansion and patches is right up there close.
Nearly every computer game released nowadays contains a few substantial bugs...it doesn't mean it's okay (IMO) to release games as such, but it happens for business and other reasons. The real release decision for companies like Paradox is not to wait until the game is perfect before release, because no game would ever get sold because none have ever been perfect...even after all their patches. Instead, the question is how close to perfect can you get ("should" you get) before the game goes to market.
I haven't played the EU4 demo yet, and I won't, and I'm sure I'll be disappointed in more than one aspect of EU4 when I begin playing it in a few days. But, I'm willing to bet that after a few patches and/or expansions that EU4 will evolve into another superb game just as most of the other Paradox games have ultimately turned out to be.