I guess I have been quite lucky. So far this year, I have flown 45-50 segments on AA and the only issue I ever experienced was being asked to pay for checked bags. This happened only once on the very first AA flight (using my DM number) that I took this year, just after they started sharing benefits. US was still part of Star Alliance at the time. The agent quickly fixed the issue, and since then, I have not had a single issue. In fact, I have been about 50% for upgrades, have always been able to land MCE seats, and have had no problems getting the free snack/drink in the main cabin when I wanted it.
Most of these tickets were booked through AA using AA metal OR US using US metal, however, I have had some that were booked through AA but on US metal and US on AA metal, and did not experience any problems as a result.
All in all, this merger has been largely problem free from my experience and I have no intention of leaving the new AA for anyone else.