Well part of the thing is how you use the card. If you use it as a mastercard or visa the merchant will pay the same rate as a credit card, in fact they have no way of knowing it is a debit card.

Also I have a few more details from Chase.

The card has a $30 annual fee, refundable in the first 30 days if you cancel the card.
It has a limit of 80,000 miles earned per year(160,000 in purchases). All purchases count towards miles except ATM withdrawls(boy if they let you do that, it would be like taking candy from a baby).

And there is a 2000 mile enrollment bonus.

So if you're a chase customer, you can get 2000 miles for 30 bucks, not a bad deal especially if you're within 2000 miles of your award.