So, my main is Pharah, followed by Mercy, D.Va, and Sombra.
I think D.Va is great. Much more fun and direct combative gameplay now. I enjoy really getting in people's faces and getting more kills, I've saved a few games/saved a few teammates already with it.
Although she is more of a DPS than tank now. I don't miss the old DM, I hardly used it and it felt too long before. This is a good adjustment.
Mercy is meh. I wasn't excited for the changes and I'm still not impressed, but what are ya gonna do. It was VERY easy for me to immediately adjust to her because I was already good at balancing battling, damage boosting, healing, and rezzing, so difficulty of gameplay wasn't an issue.
Her ult feels pretty useless (and out of character) to me. Wish the radius for her rez was slightly larger so you don't have to be RIGHT on top of the target (if you're rezzing a tank, which should be your priority for solo rezzes, they're almost always in the thick of it so it's easy to get killed).
I think she'd benefit from a 25s cooldown instead of 30 but that parts whatever, the radius is my biggest issue.
Other than that, it is what it is. I can still help my team a great amount regardless of what nerfs Blizzard has made.