Even with today's rock-bottom interest rates, if you factor in the opportunity cost of the 20% down payment over the lifetime of a 30-year mortgage loan, renting ALWAYS wins.
It is absolutely wrong to claim that "renting always wins." Let's take my house as an example, and evaluate it against some of the metrics people have mentioned so far:
The house in question is a 3 bedroom, 2 bath single-family detached home in a gentrifying neighborhood in Atlanta. I bought it in 2009 and paid about $100k. My principal and interest payment is about $500/month, PITI is about $750/month, and PITI + utilities is about $1000/month. The house is now worth almost $300K according to Zillow, or $150-200K according to my (much more realistic) best guess.
Metric 1, monthly cost: To rent a house like mine I'd have to pay at least $1500/month, plus utilities. That's twice what I actually pay (not including the opportunity cost of living in it myself vs. renting it to someone else!). Owning wins. (By the way: if I wanted to rent for the same monthly cost as my mortgage, the best I'd be able to do is a relatively low-quality 2-bedroom apartment.)
Metric 2, "price actually paid" (including interest): Assuming I take my mortgage to term, I would pay $500 * 360 = $180K total for my house, over thirty years. It's worth close to that much
now, six years in. I've already broken even! (And that's without taking into account the fact that I haven't actually paid most of that $180K yet.)
Metric 3, opportunity cost of not investing down payment: I bought with (effectively) a
negative down payment. First of all, I got a $20K down-payment assistance grant from the city. Even so, the FHA required me to have $1500 in cash at closing... which I borrowed from my parents. Then the Federal government gave me an $8K tax credit (out of which I repaid my parents). If I hadn't bought, I would have had not only fewer assets overall, but less actual
cash to invest!