Pros
As most profitable public company in the world, it is an extremely stable company. If you do your job well, you don't have to worry about losing it. This company is not going to go out of business. The compensation is very fair starting out and raises typically are greater than inflation and the market. The full benefits package is excellent, as ExxonMobil is one of the few companies still giving a pension. As a huge company, you can do many different jobs in many different places in the world without changing employers (i.e. maintain your seniority, vesting, vacation, etc.).
Cons
There is no job posting system. You have some input into what kind of job you would like to do next, however, management frequently decides for you what your next assignment will be.