The gist of it — you can keep whatever existing plan you're on now and lease a new iPhone 6 for $20 per month, and this new promo will credit you back $15 per month. This brings the monthly lease total to just $5. The iPhone 6 Plus has a similar lease pricing plan — $25 per month with a $15 monthly credit. At the end of the two-year lease period, you either return the phone or you pay off the total. Sounds like a good deal, right?
A much better idea is to spend the full $20 per month to lease a new iPhone 6, and get on the $50 monthly unlimited iPhone plan. For $70 per month (plus all those pesky fees) you'll save money versus an equivalent unlimited plan on Sprint which costs around $80 per month itself.
In any case, if you're interested in the new lease rebate deals, they start tomorrow and will be available through January 2015.