Great article in the New York Daily News written by Ted Olson and Eric Schneiderman. Here's an excerpt:
For example, after New Jersey granted civil unions to gay and lesbian couples, many employers refused to offer them partner benefits because they were not legally married. Hospitals denied them the rights of married couples, checking the "single" box - rather than the "married" box - on patients' admissions forms, thereby denying them access to hospitalized loved ones. Gay and lesbian couples were put at risk in emergencies when they traveled, because civil unions, unlike marriages, are often not recognized across state lines.
All of these legal uncertainties led the New Jersey Civil Union Review Commission to ultimately declare civil unions a failure, finding that the separate categorization "invites and encourages unequal treatment of same-sex couples and their children."