talkinghorse i totally disagree...
I wouldn't say it's normal for that kind of brake life. For example, my first set of front pads and rotors went 107k miles and the rears are predicted (by measurement vs miles data) to last over 200k miles; and, many others have reported much more life from theirs. Of course, some have reported life on the level of the 27k you're talking about here. It seems to me that life depends on the way they're used and that they really don't tolerate overheating. Overheating causes the pad material to become glassy and brittle resulting in increased wear rates.
Usually, pads should be replaced when the lining is 2/32" thick. In my case (and others too), My front pads were 17/32" when new.
Today 01:15 AM
For 1)
the outboard pads wear faster, so the measurement is not hard to do.
if your pads are wearing more on one side then the other you have caliper slide problems.
2nd
Also, since MB has designed the system such that rotors and pads reach discard thickness at about the same time, you have to be concerned about rotor thickness as well. For instance, the front rotors on my '02 with 16" wheels start life at 0.86" thick and reach discard thickness at 0.748" thick.
in every sprinter i service, not one of the rotors wore good. the pads are alot harder then the rotors. in fact in the sprinters i service with the oem rotors and pads, the pads eat the rotors way before the pads are in need of replacement.