In general, boiling vegetables can cause them to lose important nutrients, including vitamins and those that prevent cancer. In particular, Vitamins C and Folic acid are seceptible to loss during the commercial process.
Over the years, there has been controversy as to whether frozen vegetables are better or worse than fresh ones. Generally, reports show that frozen vegetables are not so much nutritionally different from fresh ones.
According to the Food and Drug Administration, based on a 1998 report, frozen vegetables have the same nutrients and health benefits as their fresh counterparts . A 2003 Austrian study found that frozen vegetables are nutritionally better than out-of-season imported fresh ones .
However, there have been some risks found with eating frozen vegetables. For example, a 2007 Australian study found that frozen vegetables may contain a bacteria called "map," which can thrive at very cold at hot temperatures, and may be responsible for Crohn's disease. 
Not all products classified as frozen vegetables can be viewed as health foods. On June 2, 2003, the USDA classified frozen French fries as a frozen vegetable .
On a side note, I think that frozen tastes better and I buy it the majority of the time.