Anyone who has picked up the 4E DMG 2 will probably remember the section on creating companion characters. To summarize the concept for those who may not be familiar with them, it's essentially a simplified mechanic used to create NPCs that effectively act as party members. They are built following the assumption of the baseline math progression inherent within the system. For example, their attack bonus will always be 4 + the character's level, regardless of the actual ability modifiers, equipment, etc. Defenses are also a base number + level (15 for AC, 13 for non-AC defenses, or NADs), though there are simple rules for adjusting the defenses (raising one and lowering others) to help the monster better fit its theme.
I just built my first companion character for a campaign that will (hopefully) come to fruition in the near future. The process was painless, and the most time-consuming part was writing the relevant data onto a large index card. Companion characters are set up with statistics that mirror those in a monster's stat block (except they have healing surges, whereas monsters do not). Since 4E monster stat blocks are very readable, concise, and intuitive this is a good thing. The fact that the old CR system was abandoned for monster levels helps to make this system work perfectly, since one 1st level monster is roughly equivalent to one 1st level PC. Because of this, companion character creation can be simplified even further in that you can simply use a monster's stat block straight from one of the monster manuals, with some slight modifications (adjusting the HP, giving it surges, and tweaking its powers a little (for example, recharge powers become encounter powers). The DMG2 provides advice on what kinds of monsters work best for this, and gives some specific examples of various levels. This will undoubtedly prove to be an invaluable tool for when the PCs do something unexpected, and somehow drag some hapless NPC into the party. BAM! Just open up the Monster Manual, make some minor tweaks, and you have yourself a fully functional party member!
It should be noted that companion characters are primarily designed to be built and roleplayed by the DM, but controlled by a player in combat. Because they have such a simple stat block, any player can pretty easily pick up a companion character as soon as he/she joins the party without it being a distraction from the player's actual character. This also gives players a little more to do in combat, which is nice in 4E since the DM is usually controlling a whole horde of monsters with the players each controlling a single character. And going along with this, the DM (who has enough to do already) doesn't have to worry about keeping track of the companion NPC in combat. Prior to 4E I used to make NPC party members by rolling up a completely statted out character. It took long, was too cumbersome for other players to use fluidly, and it was certainly impossible to generate one on the spot if need be. Needless to say, I can't wait trying out companion characters in my campaign!
The one flaw that I can see in the system is that since companion characters have such a standardized system for attacks and defenses, they don't benefit from magic items (PCs are assumed to have level-appropriate gear for the system's math to work out, but for the sake of simplicity companion characters are "level appropriate" straight out of the box). While this is great for keeping their design and influence in-game simple, what happens when PCs decide that they want to give companion characters their old gear? Makes perfect sense in the context of the game world, but it would result in overpowered characters if it were mechanically implemented. The easiest solution is for the DM to say, "ok, Carl is now using your old +2 longsword" but not actually factor it into attack and damage. If the companion is player run, they will certainly notice that the magical sword is doing nothing. Depending on your players, this might affect their suspension of disbelief and/or frustrate them since their "upgrade" has no mechanical benefits. It's simple enough for the DM to just explain how companion characters are already consistent with the system's assumed level progression, but players are used to upgrading their characters with magic items so it creates a bit of a disconnect. Overall it's probably not a huge issue, but there are some players I know that might turn it into one (which is why I brought it up). I guess the important thing to emphasize is that a companion character isn't a second character, or even a cohort (which they might be used to from 3.x edition). Very little thought should be given to the companion character on the part of the player, at least until the companion's turn comes up in combat. It's also important to emphasize that companions are DM run in terms of roleplaying, and thus actions in combat can (and sometimes should) be vetoed. The player shouldn't make the companion character do something suicidal for the benefit of the PCs. Companion characters may also have agendas and motivations that the PCs know nothing about. In fact, it's probably much more interesting that way :D