I loved the coop nature, and the arenas and round types are fun. Getting gear at the end of a set of fights is good too.
I don't like the duration for the goals though. We tried duoing the playthrough 2 quest at 50, and had to stop about half way through. No pause, no continue options. After about 3-4 hours of effort and with at least that again (since it gets harder), we were screwed. If neither of us touched a button after a round we could have in essence paused it and come back, but wouldn't have had enough time to complete it that night anyway. We had to quit without reward, and honestly, have never had a 2nd attempt.
Still good for an even challenge to just jump in and do a few rounds coop, but with it not giving you any proficiency skill ups, you get no sense of earning anything for your effort.