Alvin,
Most of my properties are on 5yr ARM's. For the most part they are either breaking even or cash-flowing. There were a few opportunities that I couldn't jump on that I'm still upset about to this date.
I don't like the idea of buying down your rate unless you're keeping it for a very long period of time. Personally, I probably wouldn't even consider it on an investment property.
I currently own a home that is occupied by a section 8 (government aid) tenant. Payments are paid by the state so they're always on time, but you must always remember that there's a reason they are getting aid from the city...
Fortunately my tenants have been good, other then their dog pooping in the neighbors yard, cops dragging away the boyfriend, tenant out drinking while leaving the kids at home and last but not least, neighbor over hearing the tenant negotiating a "price" in the back yard.
I obviously wouldn't want someone like that in my neighborhood, so now I have to evict them. This is also a whole new territory because it's kind of hard to just "evict" people in our good ole country.
Conclusion: Do your research. Good opportunities are always profitable but I hope you have enough spare time.