Pros
To me, atmosphere is everything. Every job in IT is going to come with certain common traits. But the environment that Overstock sets makes it easy to come to work everyday. While Overstock had some rocky times from 2005-2009, things are much better now. The executives are smart and know what it takes to do your job, because it was probably their job before they got promoted. The management team is very conscious of the work/life balance. Their motto is definitely "Work Hard - Play Hard". Have I worked long hours? Of course. However, I've also been told to take extra paid days off to compensate. Overstock understands that you have a life outside the office and that sometimes that takes priority.
Cons
There are not too many negatives about working at Overstock. The execs have worked very hard to filter out anyone that consistently causes "office drama", but some still exists. It's certainly much less than other places I've worked. The biggest "con" is a side-effect of this being a great place to work, i.e. no one wants to leave. Moving up can be difficult because people like where they are. Luckily, Overstock always try to fill positions from within before hiring outside the company.