I have (had) 2 phones on the AUP. My husband and I both signed up for it last year at the launch of the 6s+ last year. Recently I paid mine off and sold it. (Long story, went to Android, hated it). I now plan to get the 7+ at launch and get back on the plan.
No hidden fees, you sign up for it. They run a credit check and the phone is financed through Citizens Bank. I had the 64g 6s+ and my payment was $40.75/mo. My husband had the 128g and his was $44.91. Those payments include the AppleCare that you get with the phone. After 1 year (12 payments) you are eligible to upgrade. Nobody really knows the exact scenario for the upgrade as this will be the 1st year. Most would assume you are able to get the new phone and then will have a period of time to return your old one to Apple. But we will soon find out.
I like the AUP, I think it was good. It's just breaking down the cost of the phone, plus the AC into 24 payments and you can upgrade after 12. There is no penalty for paying it off early either.
Last year the sign up process could only be in store. Since then, they have set up the AUP to be done online. I am really hoping it stays that way for the 7 launch. Having to do it in store was a pain. You have to use a CC to sign up, the payments will be charged to your card each month. I never switched mine over but people said you could change the payment method over to your debit card or another card after set up but I was not aware of that. Not really a huge deal to have it on the CC just as long as you make the payment to the card each month to avoid interest charges.