Avantages
Job security is excellent - XOM wants to hire for life. Long-term compensation is good (401k & pension). If you get an internship and you decently well you will get more internships and/or get hired full time. Pay starts out well and increases quickly early in your career then the raises, though predictable in timing, are not very large. Strong ethical and safety performance are core values. EVERYTHING is proceduralized so at least it's predicable; once you know the system it is easy to perform well.
Inconvénients
Company evolves slowly due to very bureaucratic approach. "Old boys club"/"it's all about who you know" in full effect - transferring through the company takes luck and knowing the right person at the right time. No one ever loses their job for performance so you work with some real duds. Performance review process is done by ranking which results in back stabbing throughout the year. There are no bonuses and pay later in your career will not be as competitive as jumping to another company (though the long-term benefits with job security and the 401k & pension balance this).