what I have learned:

Will not work with the new "slim" ps2, only the older, larger ones.
Allows you to add an IDE harddrive easily to the PS2!
You can store your games on the HandDrive so you don't have to load the game disk every time.

No worries about the optical drive wearing out or the game disks getting scratched or destroyed...put your expensive disks somewhere safe.

Now that the slim ps2 is $100 new with games, this thing is overpriced at $30-40 on ebay. But if I find one at a thrift store (and a working older PS2) for a reasonable price, I will try it out.