Nearly three months ago, the Federal Reserve announced a third round of quantitative easing aimed at driving down long-term interest rates. Under the program, the Fed committed itself to buying $40 billion in agency mortgage-backed securities until the labor market improves "substantially."

At the time, many analysts, myself included, speculated that the size of QE3 and its focus on the agency MBS market would disproportionately impact mortgage REITs that specialize in these same securities, including Annaly Capital Management (NYSE:NLY), American Capital Agency (NASDAQ:AGNC), CYS Investments (NYSE:CYS), Hatteras Financial (UNKNOWN:HTS.DL), and Two Harbors Investment (NYSE:TWO), among others.

With the benefit of hindsight, we can now gauge the impact it's had on the valuations of these stocks. As you can see below, all five of the companies mentioned are currently trading at markedly lower price-to-tangible book multiples than they were at the beginning of the second quarter. Since then, in fact, all but one, CYS Investments, went from trading for a premium of tangible book value per share to a discount thereof.

NLY Price / Tangible Book Value Chart

NLY Price / Tangible Book Value data by YCharts.

Of course, for the experienced investor, this begs the question: Is it time to buy?

Before answering this, let me start out by saying that I'm no fan of the mREIT industry, and Annaly Capital Management in particular. As I laid out yesterday, there are at least six reasons that I believe prospective investors should flee from its stock -- as well as the stock of its operationally related subsidiary, Chimera Investment Corp. (NYSE:CIM).

At the same time, however, for those who disagree with my thesis on Annaly, or for investors looking at any of the other mREITs listed above, it's impossible to deny that these valuations present a seemingly attractive opportunity.

If you are so inclined, I'd encourage you to consider two points. First, keep in mind that QE3 will continue for a considerable time period, or, as the Fed put it, until the labor market improves "substantially."

And second, the agency MBS market is now comparatively crowded. This has driven up the portfolio values of mREITs, accounting for a portion of the concomitant decline in the valuation metrics charted above, and also essentially guarantees that all of these investment vehicles will be less profitable going forward than they have been over the recent past.

In other words, despite the increasingly high yields, I don't believe now is the time to buy into this sector, as there may very well be cheaper entry points going forward.

John Maxfield has no positions in the stocks mentioned above. The Motley Fool owns shares of Annaly Capital Management. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.