1) The ED requires at least 7V at the motor power input. And this power needs to be clean and have enough current. The maximum that the ED can draw is about 1.5A, so make sure your power supply can handle it. I normally use the 12V lines from old computer power supplies and it has never let me down.
2) The 'smoothness' of your steps will be dependent upon several factors. The setting of the current limit pot on the ED is the most important one. For each motor and power supply voltage, you will want to adjust the pot for the 'smoothest' operation. Note that this does not always mean 'maximum current'.
3) Does your ED seem to step differently as you touch the current adjust pot with your finger? If so, the pot is probably set in the 'open' position (between 5V and 1V of output to the REF pin - pin1 of the driver - where the REF pin is floating) and so your finger is adding noise and screwing things up. Turn the pot about 1/4 turn clockwise and try stepping again - now your finger shouldn't affect things.
4) The maximum step rate you can get the ED to do depends a lot on the motor and the power supply, but about 1 to 2 KHz is all I can get out of many of my motors. Raising the power supply voltage (all the way to 30V if you want) will help this out a lot, but remember to re-adjust your current sense pot if you do change your motor input voltage.
5) Using large stepper motors may prove a disappointing experience. With really large motors, the ED doesn't have enough oomph to do the microstepping right, and you will be disappointed with the resulting torque. NEMA-17 sized motors are just about perfect for the ED.
---- Adjusting the current sense pot ----
How do you adjust the current sense pot? Here's one way I've done it and it really works well.
A) Solder on a wire to pin 1 of the driver chip (the pin closest to the current adjust pot) or put your volt meter there. Turn power to the ED on (with the motor connected) and turn the pot clockwise until you see where it goes from about 5V, down to zero or .1V, and then jumps up to 1V or so. That point where it jumps to 1V is where the ED will deliver the minimum current to the coils (about 150mA/phase). If you then continue clockwise to 5V, that's where it will deliver maximum current (about 750mA/phase). So set the pot to 1V.
B) Now hold the direction pin either high or low, and pulse the step pin once every 20ms. It will take about 30s to deliver 1600 steps at this rate, which for a 400 step/rev motor is 1 rev.
C) As the stepping is happening, very, very slowly turn the current adjust pot clockwise. Feel the motor shaft with your hand. Notice what happens to the stepping - different things will happen with different motors. Some motors will start out real smooth and even at 1V, and then hit a point somewhere around 2.5V where the stepping becomes real rough. Some will start rough and then get smooth. Just experiment, feeling, listening, and slowly adjusting the pot.
D) As a reference point, with the stepper motor that SparkFun sells, 1.5V is about the perfect setting (I've found). Very nice smooth steps, and considerable torque.
Please post any more ideas for troubleshooting/testing EasyDriver boards here if you think of them.