I've been using the PX7 for a few days after using the PX for 2 years.
1. PX7 cups are noticeably bigger than PX cups. Sound isolation is better than the PX, but also traps more heat.
2. As other folks noted, it's much more comfortable to wear than the PX. However, I can't notice a significant difference in weight between these two.
3. As other folks noted, the build quality degraded vs. the PX. It still looks awesome, but I don't like the way buttons look and feel - they just look cheap.
4. Vast improvement in sound quality vs. the PX. Much more bass than the PX, but it sounds very organized and punchy. HIghs are clearer than the PX, which sounded somewhat veiled.
5. The carrying case is compact and well built.
I will likely keep mine and sell the PX because of sound quality improvement, but hopefully B&W improves build material quality in the next iteration.