Pros
The Postal Service has more departments than most people imagine. There is much more than letter carriers and post offices! In DC, at their Headquarters, there is marketing, finance, law, PR, strategic planning, information systems, and foreign policies. So there is something for everyone. The trouble is that you have to live/work in DC, unless you work at Engineering HQ in Merrifield, VA. But that's still pretty much Northern VA, which means it's expensive! They offer relocation, but not COLA adjustments. The benefits are excellent, just be sure to thoroughly review the plans and pick the one that best suits your lifestyle. We get majority of holidays, so that's also a plus. The hours are flexible but can vary based on your department and manager.
Cons
There are no flex hours/days, so you have to drive/metro into DC 5 days a week! They don't offer metro checks like other government agencies do! I assume because they are self funded. Sometimes it's difficult to get meaningful work, especially when you pour your heart into something only to have your manager tell you it's been put on hold. It can be difficult to see the fruit of your labor because so many other people have their hands in everything! It can be difficult to get projects that are meaningul and fulfilling to you on a professional level, especially if your manager is just a manager and is not familiar with what you specifically do. Managers are shuffled around a bit, so if you have a manager who doesn't know what an engineer does, you may be doing something that's not as fulfilling to you.