I used the ATL wifi on our layover from Sarasota to SXM in September. The initial connection required the additional step to log on to the airport wifi.

When we came back in September I had the mobile passport app on my phone and I figured I would use it to save time. My phone showed I was connected but the app wouldn't go through.
Overall the immigration line wasn't too bad. Took about 30-40 minutes to get through customs. The mobile app line had about 6 people on it and it looked like it took those folks less than 5 minutes to get through.

We had a layover of over 3 hours in ATL and after grabbing something to eat I was playing with my phone and it was during this time I found out that even though my phone shows that I am connected to the wifi, I needed to log on again to the airport wifi. I found this out by searching on the wifi for Atlanta airport map to see if there was a Dunkin' Donuts near me. It was at this time the next screen came up that asked me to log in. After doing that I retried the mobile passport app and it opened up fine.

Carol, just a word of caution if you plan on using the app at ATL, make sure you re-sign in as soon as you can get a signal, which I was able to get while the plane was pulling in to the gate. On our June trip we will be driving the 3+ hours to Fort Lauderdale to save a lot of $$$. Hopefully the app will work at FLL.