But let's skip FfH's Great Commander. Those can be used to raise a small army (one unit per three population of the city he's standing in) or make a training facility (extra production for military units and bonus xp), either of which destroys the unit. Alternatively, The GC can join a single unit (like a Warlord (S) or a Noble) giving it +1 base strength and some bonus to xp per battle, I think. And you can detach them later if you have another unit you want to level up.
FF and it's modmods expand on the GCs significantly. In RifE, the GC can raise a small army or make a training facility once but is not used up in the process. Also some buildings generate GC points the similar to other GPP sources (I think those go directly into the Combat XP counter on the military advisor screen, but I'm not sure as it isn't used at all in FfH where a GC is just one of the GP you get from a city every once in a while).
But the main thing GC do is they can lead groups of units. Initially you can tell up to three units to be assigned to your GC. This doesn't immediately do anything for them. But when they win in combat, the GC gets some XP in proportion to what they earn (I don't know what the rate is or if it reduces the amount they would have gotten). When a GC levels up, he gets promotions like Combat, March, or any of the terrain promotions, but he applies them only to the units under his command, and only when they are in his tile. He can also get promotions which increase the number of units he can command (up to three more) and the range (this I don't know off the top of my head, but I think its also three)at which he can command (at which his promotions affect) them. Unlike Civ3, you can remove units from his command any time after the turn they join and you can replace them if destroyed. GC are defensive only (strength 5) and their are issues with their speed relative to other units in the game (especially, I hear Hippus mounted units), but when I've had them, they'vs been pretty useful.
Going one step beyond that is the Bannor Chain of Command module (Bannor are a specific very military minded civ). They more or less ignore the GC, but early on can get a promotion (and I'm sure I'll get these in the wrong order) Corporal which gives an XP bonus to everyone above them in the chain of command (but has no value to its unit-it's potentially detrimental as I think it makes the unit more likely to defend its tile rather than let a higher ranked unit face peril). At level 3 or so, A unit with Corporal can get Seargent, which allows it to command a unit (I think it's just one) that has Corporal but not Sergeant promotions. Sergeants can also take any of the promotions available to GCs. At level 6 or so a unit with Sergeant can get Lieutenant which allows it to command one more unit, and any of those units can be Sergeants, and any units it commands (and any they command) get a small strength bonus (for FfH-note that most bonuses in FfH are double what they are in BTS). And so on. I think their are additional ranks available at levels 10 (Captain), 15(Corporal), and 20(General) which expand the unit's command capacity (how many units it commands), command range, and strength bonuses to all the units below it (and similarly any unit can command others with a rank promotion, but less than the unit itself). In it's current implementation, Xienwolf admits its overpowered (for one thing all of those XP bonuses stack by addition, so a Seargent is getting a 10% or 20% boost to XP earned through his follower(s), as he could be picking up command slot promos for more followers and his Lieutenant might have 2 to 5 Sergeants, so you're potentially talking about a +150% bonus that gets downright silly if you get a General with a full army), but thinks in the duration of a normal game of FF, you'd never have all the units you'd need to fill up the full army beneath your level 20 uber-commander (I think it could be solved by applying the bonus once at each rank as XP filters up through the chain of command). On the other hand the ranges are such that you'd be able to at most attack two cities at a time, you'd just really beat the snot out of the defenders. In AND, with the Surround and Destroy option on, multiple stacks of doom that the chain of command encourages, would be useful in a confined area, so it would have better application here. In AND/RoM Command ranges could/should be extended by technology (if I recall correctly General Franks led the initial 2003 Iraq campaign out of a base in Tampa, Florida). Extreme care would need to be taken with the values of command promotions since a RoM/AND game could well see armies sufficienlty large to maximize the available benefits. You'd probably want to make all the commander style promotions trickle through the ranks (in the CoC module the Bannor promotions give a different promotion to each of the followers, and that secondary promotion gives a copy of itself to its unit's followers, but the original Commander promos didn't get updated which makes most of them useless to any unit ranked above Sergeant).
So the basic idea is that you have units that are weaker than they'd otherwise be (because a level 3 Seargent has no other promos unless he got them for free and is no match for a contemporary with a couple Combat promotions), but once you got your army underway, your Corporals ultimately under a Lieutenant have half the improvement Combat 1 would given them (from his Lieutenant promo), but may be about equal to their contemporaries if either their Seargent or Lieutenant picked up any Combat-like promos (the Lieutenant is worthless in a fight with his contemporaries at this point because the Combat-like promo's do nothing for him, but he should have 2 to 4 Corporals in his tile that have to be destroyed before he can be directly attacked. If you can get a Captain with decent bonuses for his followers and extended capacity and range, then you really get to go to town. His Corporals will effectively have a free +10% or 20%, and if your promoting some units normally (not up the chain of command), look out! In RifE/FF there are some other things going on to counter this, mainly that each Civ has something they do that seems ridiculously strong until you campare it to what other civs are doing (for example the Calabim have Vampire units that get to eat city population for otherwise free xp at a rate roughly equal to the size of the city). A general counter is the Assassin unit that has an increased chance (may be automatic, I don't know how the tile defense probabilities work out) of attacking weaker units on a tile. Which would normally be a mage, but GC would get a strong preference, as would other units with Commander promos.
Odalrik also has a module that lets any unit become gain command capacity, but doesn't include the chain aspect. There was an ongoing discussion about whether units represented individuals (which makes some sense in FfH) or groups of 100s or 1000s of men (the position Valkrionn, the lead guy on RifE, takes) and thus whether it makes sense to have units commanding other units or single dudes (the GC) wandering around by their lonesomes. Also, I can't confirm whether the AI gets how to use Bannor CoC or whether it understands to keep commanders within range of their followers (though I have seen commander follower combos used by the AI), so I don't know how much touch up work is necessary for that mod, were you to go that route.
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