I thought it was just fine. Friendly staff, trees all around, convenient to the figure eight road that leads to all the Park's sights, fuel and propane available, large laundry facility and clean showers, full hookups plus dump stations, paved and level sites. A bison and deer wandered through.
I've never understood why people bash it so much, since I've stayed in some really unsatisfactory places, and Fishing Bridge was not bad at all.... I guess they expect Yellowstone to have very large sites with thick foliage and lots of space in between for privacy like some state parks. It's not like that, but to me it was perfect for busy sightseers.