I signed my S4 for cell service with AT&T México a little more than 2 weeks ago, and can now confirm the price is around an additional 9.8 USD per month (200 MXP), however, AT&T throws in a 50% discount for signing-in. So, all in all, around 5 bucks per month.
Speaking of cell service for the watch, in these 2 weeks there have been 2 instances (at least, the ones I noticed) of several hours where I could not receive calls neither in my phone nor in in my watch. I can make calls, I have cell data, but when someone calls me, it goes straight to voicemail. I have talked to at least one person I know has a cell-S4 and he confirmed this happened to him as well.
Have you guys in the states or other countries had this issue?