NPCs are psychotic, like the D said. A lot of them will attack you on sight for no goddamn reason. Some of them will scream at you to drop your "weapon" (anything in hand, including a box of noodles or a radio) and then either a) tell you to fuck off or b) rob you of random items, start walking away, then turn hostile and attack you for no goddamn reason. NPCs with flamethrowers have the tendency to shoot at enemies point-blank and torch themselves. Extra-psychotic NPCs with melee weapons will rush you and try to stab you, even if you have an energy rifle or a ticking nuke in your hands. NPCs can and will demonstrate all of these behaviors (including attempting a robbery) while you're speeding towards them at 100 km/h in a semi-trailer truck turned into a murder wagon.
The friendly NPCs are useful, but not necessarily much smarter. They can teach you skills and join you, but unless told otherwise, they tend to rush off into dangerous areas (read: the middle of a big city) and shoot everything in sight on full-auto, usually getting themselves killed. They also have the same problem with flamethrowers and self-immolation.
NPCs also tend to have a lot of combat skills. You don't quite realize how shockingly tough a living human is until you try to gun one down as they rush at you and stab you down with a bunch of knives tied around a broom handle.
Dynamic spawn makes these issues more noticeable because NPCs pretty much crawl out of the dirt. Seriously, sleep for a day or two and check the map and observe the half-dozen NPCs that suddenly showed up. With all this in mind, I actually do like playing with NPCs. Even if they make poor choices and are frustrating to exterminate at times, they add a surreal quality to the game that would otherwise be absent; there's nothing quite like a NPC shouting half a dozen curses in ten words at you then firing a tommy gun full-auto and missing every shot because of recoil. Hiring NPCs provides a degree of combat power that you otherwise couldn't get in the normal game, and make quite nice overwatch while you pick through buildings. Just don't let them see the loot, or else they might try to take it and play the "beat you there!" game.
warning: I haven't played the game in a month or so. Some info may be outdated.