on the topic of leadership and its effects, i was curious about followers as well. basically, how do you run them?
Since the feat can be pretty disruptive, you should expect that there is no standard here. DMs all over vary wildly here.
Some DMs play the cohorts & followers. Some limit them to NPC classes. Some limit them to "save your bacon" roles -- a ghost that swoops in to save the party from a TPK, but is otherwise in the background, doing little.
Some DMs just let the player roll up extra characters and play them -- full cash, real PC classes, the works.
My DM gave me a selection of NPCs from the game that already knew me and had a favorable impression. The NPCs had "real" PC classes (cleric, fighter, and so on), but they were all pre-built, and most had commoner stats or slightly better. So my job was to interview them, pick one, learn that character's backstory, and then get the character sheet and roleplay it as best as I could. The DM has overruled a few of my decisions. Sometimes I put my cohort into the thick of things, and the DM is like, "No, the character became a cohort instead of a leader so that this kind of trouble wouldn't happen, pull that cohort back."
He's mostly right, he sticks to the backstory well.
are they literally a huge gaggle of people that swarm around the PC as if they were a movie star? i'd think not, as they'd die in droves during a combat encounter, being some 6 or more levels below the PC's ECL.
That's the followers, not the cohorts. And yes, they don't follow around like adoring fans. But they're super-useful if your DM allows certain things.
My followers... well, for example I have 3 gnomes of the Expert class, and I was allowed to stat them out using commoner stats, and I hyper-optimized them for alchemy skill. The three of them are able to churn out 2 flasks of Alchemist's fire a week (on average) for free. I do have to pay a small fee to feed them, but it's way less than the flasks normally cost.
Followers make good spies. Followers make good plot hooks. Followers make good sherpas. Followers make excellent organizers of other staff in a stronghold. Also, they generally have loyalty beyond what hired staff would have. There is not writing to explain this, but consider this: you hire staff for a task, the DM can (and as many of us have experienced, will) have those staff members be cowards, stupid, etc. But rarely have I seen a DM mess with cohorts & followers, because then the DM is messing with a feat. That can get into "unfair!" accusations real fast. You know, "You haven't undermined the fighter's cleave feats, so why are you undermining my Leadership feat?"
So yeah, generally they're loyal.
the only downside would be if the PC move around a lot, would they move too?
There is a penalty in the DMG for moving around too much. You lose followers. And there is a bonus (+2 to your score, which gains you more followers) if you have a stronghold they can stay in.
My DM and I have somewhat relaxed that rule. My followers ARE in my stronghold, but the assumption is that eventually they will found some roadside shrines. This is because my character is a cleric of Fharlanghn, the god of roads & travel. It's kinda appropriate for those believers to follow the teaching of their god and get out walking.