Pro: potential for good regular income, potential for good capital gains on the property if bought in a rising area, potential for a lot of tax writeoffs.
Cons: Hassles of getting good tenants, evicting bad ones, dealing with the damage they do, repairing properties, managing day to day issues (or giving away about half of your income for a pro to do these things), etc. High initial cost of "rental mortgages" usually requiring high down payments - 1/3 and up.
I considered doing this about 5 years ago. My process was going to be to buy relatively new, fairly bullet proof (stucco houses, tile roofs, 1400-1500 sq feet, small lot, nice neighborhood) properties, about six of them. Rates were right and properties were close to rising. I made offers on two and both sales fell through in the 11th hour with the seller backing out. A lot of work out the window, and I was a little annoyed by it so I put the plan on hold. Six months later property values had risen about 15% and rates were up, and I wasnt comfortable about doing it anymore.
Five years later, those properties that I could have bought in the $150-160 range are selling for $235-260. I could have refinanced the mortgages to 40 year lows to increase my income.
So my homes would be worth about 500-600k more, and the roughly 3k of clear after tax income would more than meet my monthly needs.
A bad decision to not do it in this case.
Some people buy old crappy properties in depressed areas on the rebound, fix them up a little, rent them out to nice folks, and every week have to take a trip to one of them to fix something. That method works as well but you have to be a lot more hands on.