I've had a mixed experience. I currently have one line that was on Aio prior to the merge, and that has been fine. There was no interruption in service and all my billing info seemed to transfer over fine. My only issue there is that the line on Aio is one for someone who doesn't live with me, so when I forgot my password it was difficult to get it reset since I didn't have access to the phone.
I had a very different experience when I tried to transfer over two other lines and create a "group plan" of the three lines. First, I was told that I couldn't go through the "Add a line" function and that I should just create two new accounts and they would merge them later. That sounded odd to me, but I assumed it was because I was porting numbers rather than adding new numbers. When I tried to sign up, the website failed at the Transfer Number section. Despite having all the fields completed it would not let me progress, giving a cryptic error message about "Input cannot be left blank." I tried to start over but it then told me that my phone number was already a Cricket number and thus could not be transferred. My credit card was also charged 6 times for the order, causing a fraud alert on my card.
I spent a great deal of time with the online chat and phone support and was ultimately told that there was no order in the system for me, but that my number was being ported to Cricket. They told me that they didn't have the authority to open a new order for me to get a SIM card, and that I would have to go into a Cricket store because my number port was going to go through and I would lose my current T-Mobile service. Unfortunately the closest Cricket store is over an hour away. I told them that it seemed ridiculous that the port would go through without a SIM being provided to me but they said it was a "new policy." I ask for a supervisor who confirmed that even though Aio didn't do it this way the new Cricket does. After continuing to argue for about fifteen minutes he finally say "Oh, I just checked, you're right, the port doesn't go through until you get the SIM card."
So now I was left with T-Mobile showing that my number was being ported to Cricket, and Cricket leaving a charge on my credit card. Cricket customer service said that they couldn't refund the charges because they didn't have an order number and they also could not cancel the port process because they did not have an order number. Ultimately I had to dispute the charge with my credit card company and T-Mobile is now showing that the port "failed" so I will be keeping my service.
I still want to switch my other two lines because of the fairly poor in-building service that T-Mobile provides, but I don't know that I want to risk going through this process again.