Pros
Benefits are spectacular (401k with match and a pension). Fellow associates are bright, engaged, and some of the sharpest people you'll ever encounter at work.
Cons
Despite the vast portfolio of brands, you really don't get the opportunity to shift from, say, Petcare to Chocolate or Wrigley. Senior leadership is a revolving door, and that means tons and tons of reorganizations, shifting priorities, and inability for anyone to make a decision. The re-orgs have been brutal (imagine heavily recruiting people away from other companies only to let them go six months later after their position was deemed 'non-strategic' by new leadership). No one seems to retire from Mars - they either leave or get re-org'ed out. It's great to get Mars on your resume - and you'll make great connections - but don't expect to spend your career here.