Pros
Work-life balance is decent. Hours are flexible and people can easily work from home if they can't make it into the office for any reason. Projects are reasonable and seldom require extra hours. There is an engineering on-call rotation so people are only contacted off-hours when it's their turn. Coworkers are cool and fun to be around. The overall quality of the engineering staff is improving and this is driving a shift toward better practices and tools. The company offers significant equity to engineers, which may or may not turn out to be worth a lot after it goes public. If you love purple, this is the place for you.
Cons
Wayfair's ecommerce engine is a hard-to-maintain agglomeration of half-baked features stuck together with chewing gum. Code quality is pretty bad and there are no department-wide initiatives to improve it. The focus is on adding new features as fast as possible, the goal often being to make the site look and work like Amazon (this is openly admitted). In general minimal time is alloted for improving architecture and performance until things get so bad it's an emergency. Churning out code like this worked great when Wayfair was tiny but the company is now large and complex enough that the legacy codebase has become an impediment to progress, and management's attitude towards software engineering is an impediment to improving the codebase. Many top people do not have formal training in software management and/or have only ever worked at Wayfair and are not acquainted with modern practices in the field. They are perfectly nice and well-intentioned but don't "get" how to build great software. So, as a Wayfair software engineer you will work with bright, creative coders all working on the dull task of fixing your predecessors' follies, or rushing to hack together new features with little planning, against your better judgment, because yes, this has to go out tomorrow. Add to that continual production breakage and you'll see that doing this kind of interruption-driven development month after month gets pretty frustrating. Did I mention salaries are below industry standards? Also, free cookies and Doritos do not count as awesome perks.