In my longest running campaign ever (about 3 years real time) the PCs started out as mercs hired by the Cormyrean crown and became major world players. I never made any actual kings, but by the end of the game, most of them owned land. The game ran from level 5 to level 25 in 3e.
Human Fighter/Purple Dragon: Lord of the Stonelands, married to Azla Huntsilver, a Cormyrean noble.
Drow Eldritch Knight: Married Caladnei, the Regent of Cormyr. For those who don't know, her charge was young, so he was sleeping with the woman in charge for a long time.
Tiefling Wizard: Became the Wizard of Myth Nantar (underwater elven city) and resident enchanted tattoo artist. Attached romantically to a prominent Spellfire user.
Human Cleric of Selune: Became Mayor of Peldan's Helm (which in my game was home to a large number of lycanthropes and was the the base of operations of a major agent of Shar).
In addition the party held a number of small business establishments that they picked up over the years, an island in Marsember, and their "wheelman" NPC who was around from the beginning, Captain Kennirick of the Black Blade of Disaster, who served as a mentor and later partner / facilitator held a privateer's license from Cormyr.
I handled it in a number of ways. Generally dealing with their individual holdings was something that was done in the downtime between adventures. If you were to categorize my games with GDS, I'd probably be roughly even between the three with a leaning towards simulationist, so I wanted to know what they did in their downtime. Generally, with things such as leading bands into the Stonelands or conducting arcane studies I'd ask them what they were doing, and depending on the quality of the responses (and maybe a couple of skill rolls) I'd determine the results. Gave the Fighter a great use for his Knowledge: Tactics skill.
Monetarily, since by the end of the campaign I was having huge problems with giving out enough treasure to keep them in the appropriate range, it worked out great. Often they would conduct raids on enemy facilities or thwart attacks on Cormyr, and those wouldn't be an occasion to give out huge quantities of treasure. In these cases the reward for the adventure would be profits from their holdings, their stipend from the Crown, or credit with the War Wizards for magic item creation.
Adventure-wise, it worked out great. They became major movers and shakers, and were the elite task force for Cormyr. In my campaign there were so many enemies / bad things arrayed against Cormyr that they had no shortage of enemies. Early on it was a pirate-like campaign of privateers taking direction from the Crown, and later on they were a rapid-response force and teleportation was heavily used. They would pop in to take out the head of an enemy army, jump into emergency situations that were relayed through Cormyr's magical communication system, and then ultimately take the fight to their foes, with an aim towards solving their major problems.
They also became a target, since they were very publicly known, and had to do a great deal to protect their loved ones and holdings, and even themselves from ambushes. Even then there were some losses, such as when the fighter's evil clone killed his wife before he could arrive (she got better).
As a DM it worked well as a structure at high levels because it provided structure for the campaign. High level 3E PCs can do anything and go anywhere, and if they don't have a strong purpose for the campaign, you end up winging the entire session. However, if you provide them with a ridiculously hard challenge, you can just let them run amok and do what they do.
Ultimately it was a lot of fun.
In my 4E campaign, which has gone on for about a year real time, the party is about to be heavily involved on the post-apocalyptic world stage, having been kingmakers after toppling the Immured (a released, formerly imprisoned follower of the entropy god from 2000 years ago) who was running a kingdom and installing the rightful king to the throne. We've been on a hiatus with that one while I run a homebrewed psionic cyberpunk game based on Modern20 with my own psionics rules, but when we pick up again roughly a year will have passed and they'll be established in the Kingdom.
Interestingly enough, they were fully capable of putting one of the party on the throne instead of the king, and even had some reasons why that might be the right thing to do, but they chose not to crown themselves.
