I know a guy who owns an apartment complex. He loves being a slumlord though, and even has that stereotypical, gritty personality to match.
He currently owns only one (a 4-family unit), but says that he's thinking about buying more once he pays it off in about five years, according to him.
He's also the kind that likes to show off about how well-off he is, but will gloss over any negative details that there may be. I guess it's just the way he is, but the point is, it's also hard to gauge just how viable this venture of his has been for him.
Beyond that, I don't have any details to offer you.
Oh yeah, I also have a friend who has two mortgages, but is renting out one of them. He's not really the slumlord type, but he most certainly is the entrepreneurial type. When things do work out for him (as in, when his tenant pays on time) the rent he's earning is almost enough to pay for the mortgage. That's good for him. However, his tenant is famous for being late and not paying in full. He has already issued several eviction warnings, and even had served papers at one point.
Overall, he has confided in me that he's close to just kicking this tenant out and selling the property. Coming from a guy like him, it must have been really bad because he's not bearish in any way, shape, or form. Part of his feelings though, also had to do with the fact that this issue has also caused stress with his wife, so it's important to keep in mind that business ventures can exact personal tolls.
I'm not against people into real estate ventures though. If you read my blog, you'll see another example of one that is working out VERY well. Of course, she's much more um... intelligent about it

and she's in a better position to make such a thing work. I guess that's the point though. Regardless of what kind of business you want to get into for yourself, I guess the biggest thing is to approach it as prepared as you can be and with as much savvy as you can muster. The business example I have in my blog is actually her MBA final thesis, and she had the benefit of peer review from her professor and her classmates. Not that even that is guaranteed, but it's better than just jumping in blindly.