I think James Park hit the nail on the head. I've experienced lower end rentals in 'C-D' areas, but also rentals in 'A' areas and, in my opinion, there's no contest. Higher end areas are the only way to go, especially if you are a buy-and-holder.
Some have mentioned that it is best to invest in up-and-coming areas for the greatest returns (presumably from appreciation). Ideally, that would be the way to go. But, realistically, it is extremely difficult to truly predict (and be correct!) up-and-coming areas. Sometimes, these areas never reach their anticipated trajectory. Other times, it takes YEARS longer to materialize than anticipated. Investing in up-and-coming areas may potentially provide higher profits, but they also carry more risk. Considering the fact that many invest in real estate to mitigate risk en route to a secure retirement, I'd stick to investing in established, high-end areas where you can pick up some beautiful properties sporting attractive price-to-rent ratios.
The utilization of leverage in higher end properties--vice paying cash for cheap properties-- only helps to amplify one's returns. I can't believe one would suggest paying cash for properties instead of using a mortgage. Sure, having a mortgage increases your "risk". But, if you're buying fantastic properties in stellar areas with an economy prepared for long-term stability and prosperity, utilizing a mortgage is anything but risky-- it is essential to accelerating your wealth. And that's the main reason we invest in real estate, correct?