Microdrives are mechanical devices. They are sensative to adverse treatment. Thus said, if you treat the drive with a reasonable amount of care they are great. Most problems that I have seen posted are due to the treatment of microdrives outside the camera. Most people seem willing to be careful with their cameras, and installed microdrives, but are not so careful with the microdrive once removed from the camera.
Take your time removing the drive, be sure to handle it by the edges, put it in a secure storage case like the Hakuba or Microtech. Just a little care in handling will help ensure its continued performance.
The 8 - 1GB microdrives that I use have been excellent. Its just like any other piece of equipment, TAKE CARE OF IT, and it will serve you well.
Also remember that all equipment has the potential to fail, if you are in a critical situation, have backup for it as you would for your camera, lens and flash.
Dan.