Obama's action is clearly an attempt to circumvent the U.S. Constitution because the Senate was not in recess when he made these appointments. According to the Constitution, the Senate could not have been in recess when the appointments were made because Article I of the Constitution states that neither House can "adjourn for more than three days without the consent" of the other House, and the House absolutely did not consent to a Senate recess. It seems reasonable that the Senate should have the authority to say whether or not it is in recess. Apparently, President Obama wants to make that a presidential decision.
Obama claims he can call it a recess because the Senate was merely conducting pro forma sessions (that is, having brief meetings and not doing anything important). In fact, the Senate did pass a bill on December 23 during one of those brief sessions and the President signed it into law, so the Senate must have been conducting important business.
Listen to the radio commentary here: