Please see this page:
http://www.dpreview.com/articles/mediacompare/
Mr. Flash is some of the worst stuff on the market. Stick with Viking or Ridata.
While microdrives provide the most memory for your buck, they have several disadvantages. You must remember that the microdrive is exactly what the name says, a micro harddrive. Thus, they have moving parts, they use more power than a CF card, their shock resistance is lower than a CF and they have poor cold weather performance. In general, microdrives are more prone to failure than solid state CF cards. I think the decision all depends on how paranoid the person is about failures and how long you go between being able to dump images to another media. If you are going on a week long vacation and on the last day the drive files and you lose 1 GB of pictures...
Also, if you check reviews, you will find that microdrives slow down startup times on the camera.
Personally, I recommend several 256 MB Ridatas. They are fast and reliable and if the worst happens and your card dies, you still have all the other pictures on other cards.