DFRobot 4WD Arduino Mobile Platform Review
The whole kit was pretty simple to put together, and DFRobot throws in a couple of nice extras like a toggle switch for power, and a female receptacle you can use for your charging system. All in all, it seems like a pretty sweet deal at ~$50.
I'm using the platform with an Adafruit Motor Shield and an Arduino to control the four included gear motors. This probably isn't the optimal solution, because the yellow motors that come with the platform have a stall current of 1.5 amps at 3 volts. The Motor Shield has a max current of 1.2 amps per motor. I did some testing and couldn't get the motors to pull more than 700mA so I should be OK, but it is something to consider.
The biggest hurdle in getting the platform working with the Motor Shield was the fact that the motors would not run consistently: they would stop and start at random intervals. After half a day of research, I came across this post describing exactly the symptoms I was experiencing. My solution was to solder .1uF caps across the leads on each motor. They've worked without a hiccup ever since.
My other big concern was load capacity. I've managed to carry about two pounds with the platform, but any more weight and it just won't move.
Also, my original steering plan for the robot would not work here. I had initially planned to rotate the robot by spinning the left and right wheels in opposite directions, like a tank. Due to the spacing of the tires, the four wheels actually work against each other when this is done and the tires will eventually work themselves right off the gear motors (they just press on to the motor nubs). My solution was to only slow the motors down on the side that I want to turn in. The turning radius is greater (no more turning on a dime), but the tires won't pop off and all four motors will push the robot through the turn now.
So it wasn't quite a plug and play solution, but it does look like this will be an adequate base for my next robot. Here's a quick shot of the upper level chassis I've put together paired with the mobile platform: