Finally had to do actual work on my 3 for the first time. The passenger side rear inner pad was down the furthest, almost to metal. The front pads still look like they will make it to well over 100k miles. Amazing. I suggest everyone keep an eye on the rear pads since there are no little metal squeelers on the pads to warn you when they are about worn out. Your first warning might be that nasty metal on metal sound. That the rear pads wear faster than the fronts kind of confuses some people too.
The pad change was kind of slow since the piston retaction did not go smoothly for me. Like most rear brake calipers the 3 caliper pistons must be turned back in with some sort of spanner (I made one out of some scrap metal), not just squeezed in a clamp. You must release the parking brake cable from the caliper to get the pistons to move. They do need a little bit of pressure as you turn them to move in. The calipers themselves are aluminum FoMoCo (Ford) parts that seem to be of decent quality. Both rear discs were well within serviceable limits in thickness and runout. I scuffed them with a sanding disc and put the new pads in and called it good.
I've already got all of my 60k service parts so I'll post how that goes once completed.

Spencer

That the rear pads wear faster than the fronts kind of confuses some people too.

That confuses me. With most cars %80 of the braking is done by front brakes, therefore the fronts usually wear out a lot faster.

My 3 only has 5500 miles on yet but I've noticed a lot more brake dust on the rears than the fronts.

Anyone know why? Are they thinner cheaper pads, is there a new braking formula that uses the rears more, are Spence and I the only ones with this issue?

The front brakes on the 3 do indeed provide most of the stopping power. But as you surmised the rear pads are constructed of a lesser material. More dust . . shorter life. But hey . . . what can you expect for a $17k el cheapo car? I think for the money the 3 is hard to beat in the fun/economy category.

The rear pads wear quicker relative to the fronts compared to most cars due the braking system and the entire braking setup. Especially with brake force distribution, the car does not dive heavily under brake and there is till considerable weight over the rear wheels to allow the brakes to clamp harder before impending lock up. The braking system is world class in the mazda3, check the braking distances against $50 000 luxury cars and you will be presently surprised; Mazda did not skimp on the brakes. These brakes are very are designed for optimum performance, the only trade off is the brake dust and sometimes a bit of squeal for the first few stops in the morning, something I am willing to sacrifice.