They've only been fun to watch their last game. If they can keep up that performance, they have a real good chance at beating Argentina.
I very much enjoy watching Messi play as well, but I'd prefer my own country to win. I'll get my Messi fix outside the World Cup. Never before have I seen the Belgian people so universally invested in their national team, and it would be nice to keep this going for a while. Plus we're all just dying for a Belgium vs Netherlands semi-final (especially since we beat their arses last time in a friendly match 2 years ago).
I think the Belgium-Netherlands matches are fun, but they don't mean the same to the Dutch as they seem to mean to the Belgians. For us, our arch-enemies are the Germans. There isn't any sort of match that holds the same sort of rivalry for us as a match against them. If we get to the finals, and that's a big IF, it would be hilarious if we got to play them.
Of course we'd lose, giving us enough fuel to hate those blasted Germans for another 20 years over another lost WC finals, but c'est la vie.