Bacon Bits
Legend
I've seen it done -- done best in two player games, IMX -- but I still prefer support-oriented NPCs or henchmen rather than fully classed PCs. If possible, the NPCs should be run by the PCs, too. The DM has a lot to do and DMPCs can take up a lot of time if they're as fully kitted out as a PC. It's too easy for the game to feel like it's slipped into Mary Sue territory when the DM is describing something and then his own PC accomplishes the goal.