I say, it depends on your gaming group.
If your gaming group is cool with your playing a PC/NPC, then no worries. The most important thing to remember, though, is to resist temptation to favour your NPC while your DM... for example, putting in magic items that your character would like, or something only he can use... that's just not cool. Your group is trusting you to run a fair game, and to stack the deck in favor of your own guy is just unfair (and a reason for the next DM to slam you for it later). Also, try not to hog the spotlight. Just because you're in control of your own PC as GM doesn't give you the right to orchestrate a story centered around your character. The game is about player heroics, not NPC heroics (unless your story specifically calls for NPC heroics -- and even then, don't use your own guy as the focus).
All that aside, I agree that your PC/NPC should get an equal share of XP, regardless of his in-game status, provided he earns it.

A good thing to do is as Crothian suggests, and have him hanging in the background, but active. If your storytelling skills are up for it, it never hurts to speak in character for your PC/NPC if there would normally be a call for you to do so if you were a player (example, PC powwow's at the tavern to discuss what to do next etc).
I've had nothing but good experiences in the past for trading up PC/NPC status. Sometimes you just need a break from DMing and it's very handy to have a pre-made character that all the players already know.