I'm on the third act now, I just got a 4th person join the squad, a big tanky character. I haven't worked out the best way to use them yet, but their abilities look pretty fun.
The rewind function is great. I've been making a lot of use of that to get an idea of how my abilities work in different situations without having to worry about messing up a fight. It's also really useful when you suddenly spot a great interaction that you could have pulled off if you used your characters actions in a different order.
The writing is a lot of fun. I love the talk between missions and the characters, like the pyromancer who can't summon fire hot enough to burn people, or the traffic warlock, or the doctor who can only heal people who died so took to shooting her patients in the head.