I recall playing Birthright back, oh, probably around 96 or 97 I think it was. It was whenever the setting first came out (at least in its AD&D edition, if it had any previous ones of which I am unaware). That's a bit ago, so my recollections may be way off, but...
I remember not really being too thrilled with the setting. It centered on the concept that all of the PCs were some sort of nobility (king or other feudal leader, high priest, high wizard, whatever). Gameplay was divided up into a series of turns, each lasting some period on the order of a month or something. During your turn you went about the running of your realm and trying to keep your income up. You had to spend money in the form of gold bars (each bar being 1000gp, I think) to do things.
I fear I don't remember much about what happened during these realm turns because I was the one person in our group Not playing nobility. I started the campaign 2 sessions after everyone else, so all the convenient places in the local power structure were filled. I got stuck with being the commander of one player's army. Since he, as King, oversaw the operations of the army at the level utilized during the realm turns, I had nothing to do. Luckily there was a mechanic built in whereby you could spend 1 gold bar and 1 turn to learn a non-weapon proficiency, so I had a whole ton of them (I recall there being some discussion as to why it cost 1000gp to learn rope use and me figuing out just how much rope you could buy with that much gold).
During one of these turns the DM could thrown in some challenege, like an invasion from another country or a problem with orc raides or what have you. Either the players would send off some of their underlings to handle it or, if you were bored with the realm turn play, grab your advisors (read: the other PCs) and go handle it yourself in the form of a normal adventure.
Now, we may have just been playing the game all wrong, but I really didn't enjoy it. This is likely in part due to my position as army commander rather than one of the useful roles and the fact that the DM gave me nothing to do on my turns but learn knot tying. The actual adventure part ran just like regular AD&D, except that everyone was ludicrously rich for being low-level characters. That wasn't such a problem in 2E days since you couldn't make magic items, but with a 'turn' being a month or more (I don't recall), I can see having currency in 1000gp units at low level becoming a serious problem in 3E.