The only bad seats at Nationals Park are the ones behind the foul poles and they have kept those to a minimum. The mezzanine is good if you like being able to see the action in the outfield. The only thing to watch out for is the railing in front of row A; it can become bothersome and intrusive of your view.
Although 233 is in foul territory along the first base side, I have never actually seen a home run ball reach the mezzanine. I guess it could happen in this day and age and if it's been done, I don't know about it. So don't sit in the 200s expecting to catch a souvenier because it's probably just not going to happen.