Avantages
They pay well, especially considering the cost of living in Houston. Some people will tell you ExxonMobil is safety crazy, it's true but it has benefits. They have a high emphasis on ergonomics, everyone has great equipment for coding 8 hrs per day (sit/stand desk, ergo friendly mouse and keyboard). Fairly low stress (no strict deadline, no forced 80 hour weeks etc..) You can work a regular 40 hrs and be totally fine (but probably won't standout). Surrounded by very smart people, problems are generally challenging/interesting. You'll change jobs every 2-5 years, so you shouldn't get bored and are constantly learning. Very stable, basically 0 layoffs during these tough times were other large companies (i.e. Chevron) are firing lots of people.
Inconvénients
Not really a software shop, the organization of software development is nothing like you'd see at a company like Microsoft or Google. Sometime a lot of red tape to step through, road blocks that prevent you from using the latest and great technology. Ranking system isn't that great, lots of competition (you have to play the game right to get ahead). You have to do lots of extra initiatives to be noticed, being good at your job isn't enough. Working at the Upstream Research Company you aren't valued as much as an engineer or geoscientist (ExxonMobil IT doesn't garner the same respect).