They are OK on basic orders, but if you do anything out of the ordinary you're in for some trouble. Ive used mobile order twice and both times when I go to the window they can't get the order to process. Then they tell me I have to pay for it because it looks to them like the order wasn't paid for. I have to open my credit card app on my phone and show them that the payment was actually made at the time I placed the order. After about five minutes of waiting I finally get my food.
UPDATE: it looks like the issues with mobile order and pay have been resolved. My last experience using mobile order and pay worked perfectly.