I loved all the minor factions in New Vegas, which goes to show how well-written this game is, when you think about it. The basics of some of them (The Kings, or the Boomers) sound a bit lame on paper, but when you apply good writing to them (unlike Bethesda's inept writing team), they turn out just as believable and convincing as they are in New Vegas.
I'm a big Followers of the Apocalypse fan, myself, and at least two of my characters joined their faction, favouring science and medicine among my skills. As with all the other factions, however, they could definitely have been fleshed-out more. When you find something you like, you just want more of it, right? As for the Remnants, they definitely added a touch of a morose time-gone-by, and it was quite moving, in fact, listening to them all speak of the good ol' days with the Enclave (making the former faction seem more human as well).
One thing I often point out in Gamebryo-games is how repetitive the voice actors are - as if they've used only a few people to do all the voices for every NPC in the game - and while Obsidian has added much more flavour (and quality) to the voices overall, you still run across several of "the same person" as you adventure in the desert, and it's especially bad when you meet the Remnants. Three out of the five people that make up the group, have the same voice actor! In fact, the person who does this voice is heard all the time when speaking to a bunch of other NPC's (it's the man who voices Duncan in Dragon Age), and he rarely distorts his voice in any way, that would allow us to differentiate between the many NPC's he's allowed to voice. It's bad enough as it is, but when you're in a cramped room with five people, and three of them share the same voice, it borders on ridiculous and shameful of Obsidian not to have done something about this. Do they really only have one choice when it comes to voicing an "aging man"?
But I digress. The factions themselves are superbly written, and I manage to invest myself fully in the culture and fate of the Khans, for example. Like I said, though, I want more, more, more!