Investors in buildings with 50 or fewer units averaged an annual appreciation on their down payments of nearly 63%, the report shows. Investors in complexes between 50 and 150 units realized an average gain in equity of nearly 64%. Owners of complexes with 150 or more units saw an annual equity appreciation rate of 46.7%, which equates to an average net gain in value of about $4.84 million.

The trend toward condominium conversion influences the price trends, but it isn't the main engine behind the increase. When sales for properties intended for condo conversion are excluded, the appreciation rate remains almost the same.

"Clearly, the market has been good over the last 36 months," says David Osborne, regional manager of the Marcus & Millichap Denver office. "I'm not sure if these kinds of impressive returns are sustainable, but even at half this level, they would certainly be desirable. And given the continued strength of the apartment market, along with consistently low vacancy rates, I think that apartments are still a great investment."