It looks like any other CompactFlash card, but Hitachi's MicroDrive is actually a 1" hard disk. With a capacity of 4GB it has considerably more storage space than a regular flash memory device. At £329, it also compares favourably against 2GB CompactFlash cards.
As a hard disk, its transfer rates are slightly slower than flash memory. Also, a large drop could damage the internal mechanism. But as long as you're careful and don't need super-fast transfer speeds, it's a great product. However, with 1GB CompactFlash cards available for around £70, you'd have to be a heavy snapper to warrant this much storage.