I've been to Pittsburgh and Philadelphia for Cubs' series and Cleveland for a couple of one-off games when visiting my wife's family. Pittsburgh is great. We stayed downtown and just walked across the Clemente bridge for the games. Obviously the skyline view is great and they did fireworks over the river after one of the games. I will say that it was probably a better experience because we went during the small period when the Pirates were actually good. For Philly, we also staid downtown, but its not really walkable to downtown because of the sea of parking lots surrounding their sports complex area. The stadium itself was nice enough, although it rained all weekend, so we did not get to fully experience everything. If I recall, they have a big billboard blocking a lot of the downtown view, which is unfortunate. Progressive was decent, but I honestly do not remember much memorable about it.
I have been to 17 MLB stadiums, but that includes four parks that are no longer in use (O.Co, Astrodome, RFK, Yankee Stadium (Old)).
As a DC-area resident, I have been to Nationals park dozens of times. The park itself is pretty bland, but they have built up the area around it quite a bit over the last couple of years and so the gameday experience is much better than it used to be when they first built it. They have also improved the range of eateries in the stadium, so there is a better mix of local places and chains. To be honest, it has grown on me over the years, but I would probably rank it right around average for newer parks.