Title says: Can I use this stepper driver with a RepRap Mendel?
Link: Adafruit Motor/Stepper/Servo Shield for Arduino kit [v1.2] : ID 81 : $19.50 : Adafruit Industries, Unique & fun DIY electronics and kits
A quick perusal of spec sheets for reprap mendel motors:
http://reprap.org/wiki/Suppliers/Stepper_Motors
...compared with the adafruit specs you linked to says "likely no"; the smaller motors (nema 14) seem to be right around the "edge" (or over) what is allowed on continuous current of the drivers on the adafruit board (each bridge would drive one phase of the stepper, IIRC); the larger nema 17 motors are certainly out (they need upwards of 1 amp per phase).
Sorry - I don't think you can use the adafruit board for the mendel, unless you scale it down and use smaller stepper motors, of course (something like the Cupcake, for instance).
I want to amend something - it is true that the stepper coils are not on continuously; so you might be able to get away with using it for a while, but I think on average, over time, you would find the driver chip on that board becoming overheated. Maybe what you can try, though, is to add heatsinking to the chip, but that may or may not work for it. It may run for a while, but then again it may fail after usage during a long "run" (this would be with the nema 14 motors; with the nema 17 motors, it would probably fail rather quickly)...
Another amendment: What you might try, to avoid frying an entire board, is to buy an L293 chip, and breadboard a driver circuit for the steppers you plan to use - see if the magic smoke comes out? Then, you will have only wasted a few dollars if it fails...