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Geugon |
![Human](http://cdn.paizo.com/image/avatar/carnival2.jpg)
I have done a bit of this myself in 3.5, although mainly as a mechanism to get my fiancee involved in the game.
If you have a one-character party there is a huge problem with a string of two or three bad rolls dooming them. Also, anything even close to save-or-die becomes unworkable as a challege. Others may have work-arounds for such issues, but I think it is better to avoid them altogether.
I found it works fairly well to have the player run two characters and to run a semi-permanent NPC yourself. This make setting up CR-appropriate encounters straight-forward. My xp rewards were mainly plot-based (e.g. you just level up when I say), so I cannot comment on that. Allowing the leadership feat would probably be a good alternative to just having the NPC as well.
From a role-playing perspective, it is also helpful to have the NPC because it gives the player at least some little time for party interaction. This is important to give their character's a foil to develop their personality with as well as give you a reasonable way as DM of helping the character if they get stuck in a riddle or plot decision. I can comment on the one-player role-playing aspect more if you are interested...