Went to a Cardinals game versus the Pittsburgh Pirates earlier this season and I didn't hate these seats but they're no where near as good as the field boxes. There's just something about sitting down next to the field that makes the experience so much better. I can watch Blues and Rams games from the upper deck and not feel like I'm missing out on anything.
There were no additional seats above us. The 400 level only extends to about the pitcher's mound on the third base side. Busch Stadium is wonderful but if going for a day game, do protect yourself from the sun because St. Louis' summers are extremely hot.

When going in between each row to your seats, be very careful when walking. Our row was so high up above those beneath us, one misstep and we would have stumbled on top of their heads. But on the flip side, that also meant that we had plenty of room for our legs which I appreciated being 6'3.