I don't like these programs for a variety of reasons:
1. The phone is not yours until you pay it off.
2. When you decide to turn in your phone for these programs, they must be in good condition. At that point it is carriers (not apple, who are known to be more lenient) who decide what exactly is good condition.
3. You cannot reuse+recycle these devices until you pay carriers the remaining balance.
4. You cannot sell these devices to another person until you pay carriers the remaining balance.
5. If you lose the phone, you must pay off the remaining balance to get a new phone.
6. If you break your phone, you must get it repaired/replaced before wanting to get a new phone with an early upgrade.
The contract + sell your phone every year sounds like a better deal. Life happens... people drop, break and lose their phones at times and these programs are very unforgiving for it.
All the scenarios above are concerning wanting to get a early upgrade prior to paying off the full balance of the phone - which is the whole reason for doing it.