Yesterday T-Mobile announced that you can now buy the 16GB iPhone 5s for $0 down and payments of $27 per month for 24 months, and also the 16GB iPhone 5c for $0 down and 24 monthly payments of $22.91. Plus, you don't need to commit to a service contract. Note that since it's a holiday promotion, the offer may not be available after December 25. They're also offering $0 down payment options on the iPad. Monthly payments for the iPad Air are $26.25; $22.08 for the iPad mini with retina display; and $17.91 for the original iPad mini. In the long run, the device ends up costing you about the same as from Apple, so the advantage is not having to plunk down money up front.
But there are other advantages to T-Mobile service. Their lowest plan is $50/month for unlimited phone, text, and data. The only catch is that after you use 500MB of data in a month, they throttle the speed. Of course, you can pay more and get larger amounts of unthrottled data, but 500MB would be plenty for me. Overall, considering their low rate and their 24-month payment plan, T-Mobile is a great option in my mind.
Also, if you're thinking of an iPad with cellular data, their lowest rate for data is $0 for 200MB per month. If T-Mobile had good service available in my area, I would have bought my iPad Air from them instead of getting a Verizon device.
Interestingly, according to 9To5Mac, AT&T's CEO told investors yesterday that he thinks carriers will eventually discontinue selling subsidized phones. His comments suggest that other carriers may eventually follow in T-Mobile's footsteps and separate the cost of the device from the cost of the plan. This is typical in other countries, and a lot simpler.