I was on a DCA to BOS E190 flight with the carts, and as noted, each f/a starts with a cart in the middle. One moves to the front and the other moves towards the back.
Sitting in the first few rows in the front, it seemed like it took longer to get the drink service since they were starting mid cabin and then working their way to the front.
If they don't run from front to middle, rather than middle to front, then it would seem the premium or even more space pax may have to wait longer for the drink service.
My f/a said that they liked the carts very much.
Also there is a B6 f/a Alexia Nicole that has a Youtube channel of the same name and she mentioned she was looking forward to the transition from trays to the carts.
How if at all does the transition to carts affect express beverage markets?
Also I did notice on my 400 mile flight that most all snacks were offered, instead of in the past, they would select 2 snacks to choose from. So it seems with the carts they've expanded snack choice and availability.