To add to what all cash said:
The best part about living in the same building is that you can quickly respond to their problems. The worst part about living in the same building is that you can quickly respond to their problems. I know that sounds like an oxymoron but think of it this way. If they can catch you coming down the sidewalk and corner you it forces you to do what they need then and there, you can't even take care of things on your own timeline. They don't care that you're late to work, they don't care if you're not feeling well, etc. I know you shouldn't ignore your tenants but some times its nice to let voicemail answer the call and it is something trivial that can be dealt with the next day. And the time you leave town for the weekend or are otherwise occupied and don't get a problem resolved in a few hours time they'll freak out. :roll:
Oh, and I would NEVER in a million years invite a stranger to live in my home. Rent an apartment in the building yes, in my living space HELL NO.