I've only been at Williams for a month or so, but so far, I've loved my experience. They really do value people, and while you obviously do have to give your best, everyone's also willing to take time for friendly, casual interaction, from coworkers up to the CEO. Williams in no way resembles many of today's sweatshop style offices, for it has little direct competition, normal eight hour days, and a very low turnover rate.
To answer your questions more directly, we all dress in business casual, and can wear jeans on Friday if we've purchased a United Way Jeans Day Pass. Williams has a great community focus, and gives a lot of attention to charities and volunteering. Further, they do set up happy hours and social activities, and I've already attended a basketball game, baseball game, and arena football game with other employees, not to mention the informal get-togethers I've set up with the many friends I've already found there.
Bottom Line: Great place to be. Provides all the excitement of an energy company with a wonderful human side which so many others lack.